The registration with the RTB is valid for 4 years. That just means that if the same tenancy continues for more than 4 years you need to re-register it with the RTB. The registration fee needs to be paid again after 4 years.
That has nothing to do with terminating the tenancy.
A RAS tenancy is a 3 way contract between the landlord, the tenant, and the council.
They are not all the same, different councils have different contracts, indeed councils have changed the terms over the years so that, one RAS contract is not necessarily the same as another.
I think that you should send them a notice of termination with sufficient notice before the end of the lease. I dont see a form for that.