How Long Before Leaving Baby To Travel?

My husband and I planned and paid for a cruise for next year BEFORE we got pregnant this year. I’m expecting now and we’ve cancelled or sold all of the trips that we had for the late part of my pregnancy and the first few months of my baby’s life but I have one that I really wanted to take in July. My baby will be 4 months old and still small and susceptible to germs and things. I’m VERY TIMID about leaving him but the cruise line doesn’t let you bring your child until they are at least 6 months old. Do you think four months is too soon to leave my 4 month old baby with a close relative for the 4-day duration of our cruise? What’s a suitable age for a parent’s only getaway after having a baby? How long did you all wait to take a vacation with just the two of you?