I have been running the GRP Micro C tyres on my T4, they work indoors and were really good on the grass at the York regional too. They hardly seem to wear and are £9.99 a pair including some of the nice GRP moulded inserts
Not sure how well they will work if it gets wet though as they are the same 'C' compound as the buggy conespikes.
http://www.grpgandini.com/uk/gomme_1-10truck.html (GF02C)