I have potty trained a female Citron Crested cockatoo.
I first started out just saying "potty" whenever I saw her go no mater if she was in her cage or out with me.
Then after I had been doing that for a while when she was out interacting with me,
I would hold her over a piece of newspaper when she had been out of her cage for a little while & I thought that she might have to go & I would say "go potty" & she would go on the paper. It was very easy & painless. I trained other medium sized parrots this same way.
I believe that at 4 months your bird is still quite young....so try not to be frustrated.
I would just start by saying "potty" to him whenever you see him go potty whether he is in the cage or not. It takes birds a while to mature, at 4 months of age he is just a baby. I'd wait at least a few months to move to the next stage because of his young age.