Well, if mortals have to go at powerful supernaturals, they have to make up their lack of powers with excessive preparation. An all mortal group can actually play out that preparation in detail, since they all need that. For a mixed group, additional Fate points can act as a shortcut for the preparation, so you don't have to play a few extra scenes with one player alone every time the group has to go somewhere. +1 Refresh for every 4 sounds like it should be rather balanced.
Though keep in mind, that mortals have one other advantage that is not quite trivial: equipment. A ghoul has to pay 1 point of refresh for weapon:2 claws, while the mortal can carry a weapon:2 sword and a weapon:2 gun without the need to spend refresh on them. What's more, he can get compelled on carrying them and get more fate points that way. Armor can replace part of the toughness powers, again for no refresh costs, usually. And there are probably a few other powers that can be emulated by equipment. Not perfectly, granted, but enough to make a difference.
And yes, theoretically, anyone can get equipment, not only mortals. But "realistically", they don't. A ghoul doesn't take up a broadsword, only because it is weapon:3 and therefore better than its claws. It's a ghoul, it has claws, that's what it fights with. And I have yet to see a Fae with a Kevlar vest. It's just not in their nature, except in very rare cases (billy goats with submachine guns being one).