Sorry for the long post but please bare with me.
Short answer: NO. Why? Cause people could fend off galleons as 2 man already. A nimble ship that small which just 1 crew member can repair and bucket the water out, telling his captain to dodge a bit more while the other is fixing the ship. What do you think adding 2 more crew members will be? As long sloop got some resources, good luck sinking it.
Long answer: You could basicly have 2 ppl constantly on repair/remove water on that small ship which takes no time compared to a 3 deck galleon running up & down. I can say another issue is that you could also send 1-2 crew members on a suicide run, try board the ship and one lowers the anchor and the other focusing on killing the crew or if they brought some explosive barrel(s). Just the anchor alone, do you even know what harm that does to a single galleon?
Playing smart in a sloop should pay up. Most of the times when my friend and I lost a battle, regardless of ship type was not we got so hit and sunk instantly, but from a bit carelessness on our end getting blown up from a cannonball or pushed off the ship. Making it one man left to do everything. But if you have more crew it removes the feel of you had to play smart.
So again no 4 man crew for sloop. Maybe, just MAYBE 3 man on sloop but that would be absolut maximum and imho, overkill. No offence ment to anyone. Just wanted to get this off my chest and try explain (along with others in this thread) why this is not a good idea. If you are a "4 man premade" grouped up and on voice, you could also have two sloops well coordinated. Imagen what two good sloops could do to one galleon, same number of players, and adding 2 more crew members on each sloop, no, just no.
EDIT: A galleon with same number of crew members have alot more work to play optimal compared to a 4 man sloop, hands down. Sure has more firepower but you NEED a rather good/perfect team play, everyone talking on voice and helping each other out. A well played 2 man sloop can sink any half competent 3-4 man galleon. Did it myself with my mate on our sloop, and by that I don't mean to sound like some elitist/tryharder snob. Tho when we faced good galleon crews, that became far more difficult, but still not hopeless. Sorry for the long reply and grammer but english is not my native language :)