We took our son on his first cruise when he was 20 months old. Looking back, it was not a lot of fun for any of us. You've got the diaper thing going, the naps, the crying for no apparent reason, the need for a stroller, etc. My wife was a wreck by the end of the cruise and I wasn't much better.
I'd really recommend holding off on the cruise with a 4-month old. But that is me.