I thought all GRX was Hydraulic. I could be wrong.
I run 4720 (hydraulic) shifters to Shimano SLX post mount hydraulic calipers and they work fine.
Going to depend also on the front, but with a double front, the GS version of the 4700 RD gives you official max 34t. So it will probably work on 36t as long as you don't push the front limits as well.