Keep going with the night feeds - there is a lot of evidence to say your best milk is made at night!
Catch-down growth is where a baby is finding their natural position on the growth charts. Lots of things can artificially raise birth weight (including IV fluid quantites given to mothers during labour, being overdue, maternal diabeties or elevated glucose levels during pregnancy etc.) which are gradually adjusted for over time.
Do you have a breastfed baby growth chart in your red book or is it a bottle fed one? There are two different charts and comparing a breastfed baby on a bottle fed chart can be depressing!!!!
A couple of things you might want to think about for your own peace of mind - do ignore if not relevant!!! Are you using a dummy during the day? If so, it might be worth taking it away for a while as they can mask feeding cues and the sucking action can tell the baby that he is full when he isn't? Are you truly feeding on demand? It may be that just having one more feed in the day will be something he will happily take and will just give him that little boost?
The other thing that I remember happening at around 4 months was that my LO became much more inquisitive and was always looking around at what was going on. I found I had to offer milk more often as she was forever getting distracted looking at things, especially when we were out and about. I found that I had to go and find a quiet place to feed to make sure she really did have the feed she wanted.
It is worth going back to clinic in a few weeks and weighing again, just to show yourself it has all been good :)
You are doing great! Keep going!!!