Four foot swells are almost as flat as they come in the ocean. Also its not just the swell height but the interval. Fifteen foot swells would be ok with a 30 second interval! Last weekend it was a five foot swell at 5 seconds. That means from wave top to wave top in five seconds. Very short, if that tri hull has an open bow you need to be very careful. Poke one wave over the bow, all that water will end up in the boat...and now you have problems. Watch the wind coming up in the afternoon too. Wear your PFD! If the wind is ok and the seas below 5 feet with a long interval, no problems go for it and have a great time!