for most class 3's and some 4s. It just depends on why it's rated class 3 or 4. If it's because of a lot of rocks, etc... in the rapid that you have to maneuver through, then you should be fine. If it's a 3-4 due to large hydraulics? Could be iffy. We typically took 18-22 foot boats on big water rapids when I was a guide, and even then we would have to get the passengers to punch the tube down as we hit some of the bigger waves/holes to make it through.