Since the pumps are supplying GPM's at a certain rpm, you have a combined flow.

The pressure will be dictated by the load on the pump.

Say you had a combined flow of 10 GPM's, and a hyd component that can develop about 3000 psi, then you will need 21 HP for that situation.

Now, the 4 GPM pump drops out, that leaves 6 GPM, then the HP requirement now becomes abut 12 HP.