I try to go back and forth between 4 and 5, and have a lot of trouble with going to the 5 after playing the 4. (Going to the 4 after playing the 5 is easy.) Maybe this is because I'm not a very good player in general. The choir I play for has delivered me a bunch of music that really clamors for the 5er this quarter, so I'm going with it exclusively and have come to the conclusion that I'm a really lousy 5-string player even when I'm not trying to mix in the 4. Sigh.