Yesterday, tried the F101 with RS4 7.8 and Tectonics 38 goldwing fin, in choppy conditions, worked great. Did 30 knots on it. Earlier this week, tried the 101 with RS4 5.4 and RS4 6.2. The 6.2 was great, but 5.4 felt too small - but the fact that my home spot is very choppy could also play a part.
So in conclusion, for me the futura 101 works great with RS4 6.2-7.2-7.8; can also be sailed with 5.4, but prefer a smaller board with that size.