I assume that Mikoto is rank 3 and Mugino rank 4 because she can pseudo-fly and Mugino can't. Mugino's power seems much stronger than Mikoto's in terms of pure destructive capability. Mikoto's railgun does about the same amount of damage, but Mugino can do it all the time and doesn't need anything ferro-magnetic to fire. It kind of bothers me that flight seems to be the deciding factor when someone's rank is considered increased. Accelerator was perceived to have powered up after he started making those tornado wings, when he could have always just altered his own vectors and jumped as far as he wanted. Mugino doesn't appear to be able to fly, so she's weaker...riiiight. :/
Guess that's another so-called toaru trademark, arbitrary rankings.

There was also the line about how the two can defend or deflect each other's powers, so they operate on the same basic principles. I suppose where Mikoto moves electrons to create and manipulate electric fields, Mugino brings them to an extremely excited state to make free-electron lasers. Looking forward to their fight.