with Losi and Proline making softer tires and discontinuing their harder tires their foams have become bigger and harder and the 'good' 'cloud' foams that Losi do aren't available anymore
Losi 'firm' foams are OK if you want to pack out your tire, Proline don't do anything that works in 'UK' tires.
with regard to 4WD fronts, best thing is to put the same in the front as you do in the rear, just trim them to the width of the tire, not the wheel.