Return Current of a 200MHz signal sandwiched between GND and split power planes
Caps at the split will not "encourage" more (high-frequency) current to flow in that plane, because the whole point about the problem with splits is that at the time the current sets-off into the 'bad' plane it doesn't yet know about the split - it's when it arrives at the split that the problems start.

But if there really is a ~10:1 difference in spacing between L3-L4 and L4-L5 then I doubt it's a big deal, as most of the return current will be L5 anyway.