Hello mate,
I was running the yellow T4 today! As you said there's two ways you can go as the B4 and T4 run in different classes. The buggy class is a bit more competitive, with more people entering. The T4 will run in the truck class which is always a mix of Stadium trucks and Short courses.
Availability of parts is fine, the T4 shares a lot of the components from the B4 which are still easily available. The other parts the T4 used are from the SC10 and getting bits for these is no problem. If you need tyres or anything too Rob from SouthwestRC who was there supporting us there today can order tyres in for you
Yatton do get a turn out of trucks but not always and sometimes it's just a few, the best turn out for trucks is at WORM where we have around 10 cars running from time to time.