(at least here in germany) there always guitar cabs as part of the backline. i've actually never taken my own cab on stage so far. the details about the backline that should be in your contract with the festival 
on bigger festivals there are (mostly) functional monitors, so you won't need your 4x12 to hear yourself like when playing at the local bar :lol::lol: About the only reason I can think of for going with a 4x12 instead of a 2x12 is you might be able to hear yourself better on stage, but chances are they'll have a decent monitor setup there anyways.