What size circuit breaker do I need?
With #4CU you can have up to a 90A feeder breaker. Even at 105' a 90A feeder is FINE. 

The sum of the breakers is a completely meaningless number, you need to consider the actual load you will have at any given time. Think about it, a typical 200A panel might be filled with 400, 500, 600, or more amps worth of breakers. 
Better yet do a load calculation, which in an installation like this is pretty hard to pin down.

IMO there is no reason at all to go with less than a 70A breaker. If it were me the 90 would be in there already.
Your wire will handle it, why limit yourself???

Actually, re-reading it You are saying you want to use a GFCI breaker for the feeder? If so, WHY?? That is an expensive breaker that is NOT needed. Simply use GFI receptacles where required at the point of use.

If this is the case then I'd go with the biggest GFCI breaker I could find under 100A, which is most likely 60A.