I also think it is better to try 3.5 h for a while, then you can see if he would start to have the same issues again and need to extend more. Anyway, I would keep all transitions slow, extending of 15 mins every few days. The uninterest in feed started for my DS at about 3.5 months and it was linked to the transition for extending feeding times. I was not expecting it, so it took some time for me to realize it. In the end, I understood he was not really hungry because I tried giving him a bottle (after he refused the breast) and he didn't want it.
It was in the same period that my DS started to eat very quickly, it took only 10-15 mins to completely empty both breasts, before it was taking 30 mins or more. I have to add that my LO was a big baby, he was well above the average in weight. I know every LO is different, you have to find what is better for yours.