hi there
your situation sounds very similar to ours. My LO just turned 7 months a few days ago, does 3h20 for 1st and 2nd A time, and takes a 2h15 and 45 min nap (of which I usually wake him). However, we tend to get NWs and EWs if we go much beyond his A so we are holding steady with what we have for the moment.
I would say if he can handle 4 hr A to perhaps try it for a few days and see if any NWs pop up. However I would think there may be a possibility of OT building up, and might manifest itself at some point. But I guess it couldn't hurt to try??
I am already looking forward to the day of 1 nap - life will be such bliss