I've seen a Ryobi 31cc pull a 25 pound Cub on floats out of the water just fine. Are you using CDI ignition? You should and remove all the baffles from the stock muffler or use a Jag Engines muffler this will help give you a bit more power. Dynaflyte (Great Planes) tends to overbuild which leads to a heavier than necessary airframe. You'll be in the ballpark for sure, takeoff speeds however may not be blinding let her climb out on her own don't pull it up, she will build speed slowly it will be fine. Use a good balanced sharp APC prop NOT A WOOD prop. Not sure of the flight weight of the 4* but my Gas hog bipe is 16 pounds and she flies just great with a Ryobi 31.