Poor little guy.
Jackson, my almost-3-year-old, just went through a bout of pancreatitis in July. He had to stay overnight hooked up on IV's, etc, but his was brought on by a lot of fatty people food he got into (chocolate cupcakes, sausage from the pizza, etc). How did your vet make the diagnosis? I did a lot of reading and saw that a lot of vets over-diagnosis pancreatitis, especially if they see on the blood work that the lipase levels are high, they automatically just say pancreatitis. It does seem a little odd to me that a 4 month old would get it, but I know it is possible.... my vet told me she mostly sees it in older dogs or overweight dogs. My vet, after a week, sent out for a cPL test (a canine pancreatitis specific test -- something or the other) and it came back in normal range, aka negative for pancreatitis. If I had known better, I would have requested this in the beginning... so you may want to ask your vet about it.
I was sooo super worried and concerned about Jackson, you can find my thread somewhere, I was a wreck, haha. But things simmered back down to normal and now just as a precaution, I only give him kibble and wet food for his meals (it was difficult finding low fat canned foods) but the key is keeping their fat content low. I did put Jackson back on regular food (Castor & Pollux Organix Weight Mgt. to start). We are actually experimenting right now with California Natural grainfree to see if it helps with his eye stains, so far so good. Most dog treats are really high in fat, I've come to learn, but I did find Buddy Biscuits (the soft kind) which list a minimum and a maximum fat. Also, Jackson has been okay with Cheerios as treats too. However, I wouldn't be giving him any thing other kibble for a while. I only used the RX food for the first 3 weeks or so to calm his pancreas and tummy.
A lot is still unknown about pancreatitis. Sometimes it just happens... there have been dogs who get it and they NEVER get fed table scraps, only kibble, etc. It's so confusing, to me, still. But there are a lot of knowledgeable members who helped me out during my time of "crisis".
Good luck!!