I can't imagine that eating every 4 hours vs. every 3 hours is causing constipation. Poor little thing
I of course could be completely wrong though
If the 4 hour EASY is working for you then don't change anything,
Keep offering more water...in between every feed.
It could just be his body going through a change and it's coinsiding with the move to a 4 hour EASY
My dd goes through stages like that all the time.
Melissa