Have they plotted a centile curve for him? Generally, they want to see that babies grow on average about the same rate over time, but there will be slower periods in growth just before a big jump. I think there is often a big jump around 4 months, so it could be he's just coming up to that. But really, it's all about the average, so if you have a centile chart for him, you can see how his growth has followed the curve over time. Most babies stay on average around about the same (e.g. if born at 50th centile, they are still 50th centile at 4 months), but some do change (my daughter was born at I think 25th centile and was around the 60th at 4 months, she lost a lot of weight after birth, down to 2nd centile, so the rebound from that brought her up above where she had been). I think the only concern would be if there is a huge drop (e.g. born at 70th centile and then at 30th at 4 months suddenly). I would expect he may be eating less at daycare. I know my daughter did take less when she was away from me to begin with. If he's feeding on demand as much as he wants and whenever he wants it (I would just reiterate with your daycare that this is in fact what they're doing), you probably don't have anything to worry about and whatever up and down there is will even out eventually.
EDIT: Actually, I just dug out my centile chart and had a look. 7lbs 9 oz would have made him just below 25th centile at birth and 14 lbs 12 oz would make him round about 40th centile at 4 months, so he's actually gone up rather than down in centiles. That's perfect and not something your doctor should be stressing you about. If they have concerns in the future, tell them to be plotting the centile curves and make sure they're doing it correctly. They should be doing it anyway. I know here they do it in our children's health record book, which we keep at home with us, so we always see it, but they should at least be doing it for their own records, even if they aren't given it to you.