Horn vs Strings--Another Dynamics Question

With the cellos, violas, and horns each playing their own harmony part mf, is two horns or four better for balance? (I gave the violas the part that I want to carry almost, but still not quite as loud as the other two.) The horns are playing upper middle register if that makes a difference, and the violas and cellos, pretty much their respective middle registers. All three are playing mostly quarter notes at allegro tempo

Two horns are sufficient; but four will work well also. Horns tend to blend well with almost any other instrument, and so will not overpower any other parts unless you tell them to.