Be very carrful using a plate rating to decide what your boat is capable of, either way.
Part of the rating is what equipment is carried as well as the catagory of boat and there can be some extream swings each way.
Keiths table is spot on, but my Ribcraft 4.8 is Cat D,
D - Sheltered Waters. Small lakes, rivers and canals, wind conditions up to and including force 4, significant wave heights up to 0.5m.
Tell this to the RB4 people !!.
Also wave height for the B & A often refer to open ocean wave height, which often roll and seldom break until it get very windy.
A 4M rolling wave may be no prob, but if its breaking in shallow water, that may be a major problem.
Keep the plate rating in the back of your mind and start of easy and decide what YOU, YOUR CREW can handle then think about the boat, bet the boat wins.
Also check if the equipment is still there for Cat B, not sure but I think things like 406MHz EPIRB, Liferaft etc are requirements.
Maybe someone knows what the actual equipment requirements are.
It is better to decide to give today a miss and play tomorrow rather than think the boat can take it and never play again !!