First of all, the Fate point pools available to a character would alter the balance significantly, so let's disregard the use of aspects and fate points for now.
Let's assume that on any given skill involved in a fight, a 5-cap character has a +1 advantage over a 4-cap character. When on the offensive (ie. when a result of zero favors the attacker), the character with the one-higher skill would come on top at least ~75% of the time. When it is the lower character's turn, he only has a ~40% chance of coming out on top.
If we take these numbers represent the resource cost of an attack to the target, it becomes clear that you need two 4-cap characters to cope with a 5-cap character. (or two 3-cap characters vs. a 4-cap character)
With two shifts of difference between characters, the numbers become ~85% vs ~25%, so three 3-cap characters could somewhat be handled single handedly by a 5-cap, while four of them would represent actual trouble.
Of course, using smart tactics and efficient application of aspects and fate points, it is entirely possible for two 3-cap simpletons give a very hard time to a 5-cap hero.
A simple stress-per-exchange analysis based on the assumption that characters do nothing smart but whack at each other continuously indicates that a single shift of difference between characters equates to a threefold potential in hurting the other. Of course, in normal gameplay, three 4-caps would easily beat the excrement out of a 5-cap by stacking maneuvers.