I really like the Active Time Event in the Susano and Shinryu. It gives an intense sense of the party working together. Perhaps we can have ATE LB for LB4.

FFVIII and FFX has limit breaks where you have to press a sequence of buttons or hit the button at a correct moment. FFXII has chained Quickenings.

So depending on how well it's done by the party, the effect can be 50% to 150% of a regular LB3.