Check what your insurers require for the price of the pony. Sometimes they need a 5 stage if you pay over a certain amount. But to be honest if it is a ridden pony I would get a 5stage, it will be worth it for peace of mind.
I had a 5 stage for my boy and personally always would when buying a horse.. But that's just because I would be riding and doing more than perhaps a broodmare. Can't remember the cost though xx