Did you get him/her from a private-breeder or from a pet shop? Did you get a "hatch-certificate" with him that states his hatch-date?
The question really is how sure are you of his age being around 4 months old? It's probably just his way of interacting with you/bonding with you, or if he's actually closer to 6 months old it could be his sex-hormones kicking-in and he's trying to regurgitate for you..Is he bringing anything up when he does the head-bobbing? Usually when they regurgitate they bring up tiny bits of solid white material, that's the dead-giveaway that he's regurgitating for you and it's his hormones...
***What you need to be sure of is that he is fully-weaned and he's eating solid food appropriately and that it's sustaining him...Unfortunately we get tons of people who bring home young parrots and they were lied to about their age, and they aren't actually anywhere close to the age that they were told that they are....And when this happens and they aren't fully-weaned, that means that they aren't yet eating enough solid food to sustain themselves and the breeder just sold them too soon and they should still be eating hand-feeding formula several times a day...So that's what you want to be careful about...
What is his regular, daily diet that he came to you on?
****I HIGHLY SUGGEST that you run out and buy a digital kitchen scale, you can get one at any Walmart, Target, etc. for around $15-$20, then you need to start weighing him every single day to make sure he's not losing weight. You have to weigh him every day at the same exact time, and the best time is first thing in the morning AFTER he poops but BEFORE he eats his breakfast. Weigh him and then write down the date, time, and weight in a notebook to keep track of it...If you weigh him at exactly the same time every day and see that he's continually losing weight each day for a week, then you've got a problem...If his weight is stable or goes up a gram or two and then comes down a gram or two, etc., then he's fine...You also need to keep careful track of exactly how much of his solid food he's eating each day by measuring out his daily pellets/seed-mix, his fresh veggies, etc., and then keep track of how much is left after he's done eating...Often when they aren't fully-weaned it looks like they're eating a lot of their solid food, but when you actually keep track of it you find out that they're simply playing with it and throwing it around, and aren't actually eating much of it at all...