You"ll be fine, just don't slosh it around. Buy a 5 gallon carboy for next time.
Or, better yet, brew a six gallon batch next time.
Edit: actually, though, c.n. budz is right; more traditional is a larger primary and smaller secondary. I have 6g primaries and 5g secondaries and brew 5.5 gallon batches. Tends to nicely fill up the six gallon to minimal head space and does the same when I rack to the secondary, leaving some of the beer behind with the trub.