I'm pretty sure McBride and Jody Hill were the ones that ended Eastbound, HBO had to talk them into doing a season 4.the final season of How to Make It in America drew a weak 0.54 average rating, however Girls is doing the same thing right now in 2015. It has 4 episodes left this season and is currently pulling 0.59, yet like @FlyRy mentioned HBO already locked it in for season 5
Eastbound & Down is another example. it had better ratings than Girls did in each of the two years they were on together yet HBO was quick to cancel Eastbound.
And HTMIIA never got the critical acclaim that Girls gets.