OK - this has been a long time problem with the ct4's.. But after some research i have found replacement belts for all 3 belts on the ct4s.. Some modifications may be needed as shown previously in the post.. The belts are -

Rear Belt
ONFA NZ10 - stock # 38070
Length - 177mm - cen part - 177mm
width - 4.5mm - cen part - 3mm
Teeth - 59


Side Belt
HPI # 87007
Length - 174mm - cen part - 171mm
Width - 4mm - cen part - 3mm
Teeth - 56


Front Belt
HPI # A726
Length - 405mm - cen part - 402mm
Width - 5mm - cen part - 3mm
Teeth - 135


I havent actually received the belts yet, but i've ordered them. You can buy the belts from here
http://www2.omnimodels.com/cgi-bin/woi0093p?&C=CLF
http://www.towerhobbies.com/listings.html

The HPI front and side belts will have tensioners so they should take up some of the slack given by the slight changes in lengths between the Cen and HPI belts. The qaulity of the belts are also 10x that of the CEN belts! Look at the last photo's teeth!!

The rear belt has to be spot on. The Onfa belt is exactly the same length as the Cen part - so in theory - this should work also! Im not sure on qaulity - but from the photo it doesnt look too bad... Anythings gotta be better than those Cen rubber bands!!!

I really hope this is the end of the ct4s belt problems!!