So you didn't actually breed these guys, but were given them when they were just hatched? And you've hand-fed them since? Good for you for being able to do this successfully with no experience, as it's not an easy thing to do...
As far as the hand-feeding formula goes, Monica is exactly correct, it's not meant for adult birds, and at 5 months old they shouldn't be eating it anymore...And while you never, ever want to "Force-Wean" them, which means just suddenly stopping the formula feedings and giving them nothing but seeds/pellets to eat, it doesn't sound at all like you've done that, it sounds like you have absolutely "Abundance-Weaned" them...
Here's the magic question: Are they actually crying/vocally whining at nighttime for the hand-feeding (what we call a "comfort feeding"), or are you just automatically offering them the formula feeding at nighttime? And also, have you been weighing them on a digital kitchen scale every day and keeping track of their weight? Is that one nightly feeding the only formula they are now eating? And at what age (or how long ago) did they stop getting daily formula feedings?
IF the nighttime formula feeding is the only formula feeding they are getting, and they are eating their seeds and fresh veggies/fruits throughout the day, every day, and if they've only been getting that one nightly formula feeding and no other formula feedings during the day for at least 3-4 weeks, then they are actually fully-weaned and have been for some time, regardless of whether or not they are actually "asking" for that nighttime formula feeding.
If they are not vocally asking for that nighttime formula feeding and you're just offering it to them, then you can just stop it immediately. However, if they are still vocally crying for their nighttime "Comfort-Feeding", then you need to get them off of it ASAP, and you need to start the process immediately. And the way you do this is to look at the amount of formula that you've been feeding each of them during this feeding, and then starting tonight you simply reduce that amount by a quarter for each bird. What you have to realize is that they are not asking for his "Comfort Feeding" because they need the feeding at all, or even because they are at all hungry, in fact, at 5 months old they are likely already full of seeds by the time you give them this feeding at night. They vocally ask for this "Comfort-Feeding" for just that, comfort. And it's up to you to now stop it, for their health.
Baby bird hand-feeding formula is loaded with calories, fat, carbs, sugar, salt, vitamins, minerals, amino acids, etc. in extremely high quantities, much higher than are needed by a 5 month-old bird. So what tends to happen to birds who are fed hand-feeding formula into their adult age is that they can develop serious health issues, just like an adult dog being fed puppy-food...Everything from Fatty Liver Disease to Kidney Disease due to the extremely high protein content, to Gout due to the protein content, to Vitamin Toxicity due to the extremely high dosage of fat-soluble vitamins in the formula, etc. And it's much worse for captive/pet birds who are not flying/exercising all day long, every day. Those extra calories, fat, protein, carbs, etc are just increasing the size of the fat-stores every single day, not only on their bodies, but in their livers, their kidneys, around their hearts, etc.
So again, if your birds are not vocally "asking" for this nighttime "Comfort-Feeding" of formula, but rather you've just been giving it to them because you thought they needed to have it, then you can simply stop giving it to them. And if you don't give them the comfort-feeding tonight and you find that they do in-fact start vocally "asking" for it, then you'll need to do the same thing that you'll need to do if they have been vocally asking for it...So if they have been vocally "asking" for the comfort-feeding, then starting tonight you need to reduce the total amount of formula that you've been feeding each of them by a quarter...So for example, if you've been feeding each of them 3ml of formula at night before bed as a comfort-feeding, then tonight you want to give them only 2.25ml (honestly, I'd round-down in this situation, and only feed each of them 2ml starting tonight)...Again, they aren't vocally asking for this formula at night because they are actually hungry or because they need it, it's a mental/psychological thing, and what you should find is that even if you only feed them each 1ml of formula tonight, they will most-likely be happy with it and not make a peep; however, I always like to make 100% certain that I'm reducing even the comfort-feeding gradually...Just keep in-mind that this is not the same at all as "forcing" a 2 month-old baby to go from 4 feedings a day to 2 feedings a day, and just letting them scream. In that scenario they would be screaming because they are hungry and their bodies are starving for the nutrition. In the case of a 5 month-old bird who is eating seeds all day long every day and who is then screaming for a nighttime "comfort-feeding", they are neither hungry nor starving for anything, they simply want the attention and the comfort before they go to sleep.
So the idea here is going to be to reduce the amount of formula you are feeding them at nighttime a little, and at their age, I would be reducing it every other night or every two nights. A good rule of thumb in this situation is that if you reduce their comfort-feeding and they make no fuss and go to sleep, then you can go ahead and again reduce it the next night. At their age, I'd be setting your goal to be totally eliminating that nighttime comfort-feeding and no longer giving them any formula within a month tops. They really do need to get-off of the formula, for both their physical and psychological health.
I also agree with Monica that the cage you have them in in that photo is far too small for 4 Quakers, it's hardly large enough for 1 Quaker (unless it is their sleeping-cage)...I don't know what you're intention with these 4 is, if you're planning on keeping them or giving them away/selling them, or just keeping one, etc., but at 5 months old you need to keep in-mind that they are approaching the time where hormonal behavior is going to start, and since Quakers must be DNA-tested in order to know their gender, the last thing you want is for them to start mating and in-breeding. Quaker's are extremely playful, energetic parrots, and they need a large cage with lots and lots of different toys and foraging activities at any one time to keep them from becoming bored, and to keep those beaks busy. My Quaker is now 3 years old, and she has the same size of cage that my Senegal Parrot does, and she uses every bit of that cage all day long, every single day, climbing and swinging and jumping, and also that size of cage is needed just to house all of her toys. She has at least 7-8 different types of toys in her cage at any given time, usually more like 10+ toys at any given time, and I try to rotate a couple of new toys into her cage at least once a month. She also has several different foraging activities in her cage and on her play-stand, she has a plastic ball that i fill with treats and that she has to work at to get the nuts out of, and she loves it. Quakers are extremely intelligent and busy parrots, if you've ever seen the extremely intricate housing developments that they build in the wild then you understand. And Quakers are also extremely LOUD, and not at all shy about letting you know that they are bored, displeased, hungry, etc. So it's extremely important to always make sure that they have tons and tons of space, toys, foraging activities...basically, Quaker Parrots need a job to do at all times.
As far as getting them to eat pellets goes, before worrying about this you need to get them off of all hand-feeding formula. That really does need to be your main priority right now. Once you've completely eliminated that nighttime comfort-feeding, which again shouldn't take longer than a month at most, then you can start transitioning them into getting used to incorporating pellets as a staple of their daily diet along with a seed mix...For now, please make sure that whatever seed-mix they are eating is a healthy, fortified, VARIED seed mix that is low in fat, specifically contains no sunflower seeds, as Quakers are extremely prone to Fatty Liver Disease, and that contains an array of healthy seeds, grains, legumes, nuts, dried veggies and fruits, and different pulses, along with some different things like dried peppers to keep them interested. I feed both my Quaker and my Senegal Parrot either Sunseed Vita Prima for Conures with no sunflower seeds, Tropimix seed-mix for Conures, or my Quaker's favorite is Higgin's Vita-Seed California Blend. And then plenty of fresh veggies, which is usually easily fed by making large batches of "Chop", putting them in small baggies with a day's serving in them, and freezing them. Then you can just pull out the baggies of Chop for the next day the night before and put them in the fridge to thaw. If you go to the "Diet" forum here you will find endless Chop recipes. My Quaker likes it if I make the pieces of veggies in her Chop large enough for her to hold in her foot and eat that way, but not too large. Quakers usually like to hold their food as they eat it, not like some of the other smaller parrots who like to pick at their food and not hold it...
Just a lot of small white feathers? No actual bare patches on the birds? 
