4 slots per job would be nice, though i think with the amount of explorer jobs out there 5 would be more appropriate to accommodate different situations (maps,bosses,etc).
as for the ability to use one buff and one attack - that might be a bit broken as it will allow phantoms to use final cut with any other mobbing/bossing skill from 4th job that belongs to explorers, which means a huge boost in damage output.
a solution to that could be to limit the skills that you can use with said stolen buff (sort of like how we cant steal shadow partner).
