If he is getting distracted, have you tried feeding him in a dark quiet room, so that there is nothing else that can gt his attention, and has to concentrate on feeding?
As they get older, they get a lot more inquisitive, so they want to constantly see what is going on.
Also with regards to teething, your lo may not actually be teething, but his teeth could be moving up in his gums and this may cause him some discomfort.
Hugs I know how hard it can be when they are not feeding as much as normal. Things will get better soon, I promise.
Also at around the 4 month mark there is a big growth spurt, so he may start taking more soon.