David Keeley, for one, races a four. Any success he's had on it is more due to his riding abilities rather than the bike. I hear they can be made pretty quick for their class but a considerable amount of cash would need to be spent. I never tried to build one... my impression is purely a result of observation.
I have one that I bought from a friend, it has been sitting up in my Shop for about 2 years I am not sure what to do with it. Thanks Jim