On the basis we're running 5.5 FDR indoors in the Quick Silver class - I'd start with approx. 7.5 FDR (accounting for larger wheel size and drag from the grass surface). I understand the internal ratio of a B4 is 2.6 and the stock spur gear has 81 teeth; therefore try a 28 tooth pinion as a starting point - this gives you 7.52 FDR. Dependent upon the track and conditions, you might want to go up or down a tooth on the pinion.
Cheers
James
Try 'Quick Silver', the Amazing Silver Can Powered Touring Car Class at HRCCC