Does Improved Initiative (+4 on initiative checks) stacks with Superior Initiative (+8 on initiative checks)? Is it possible (if it's a good idea) to retrain Improved Initiative to Superior on epic tier?
Improved Initiative and Superior Initiative do not stack.
Both feats give a feat bonus to initiative, and bonuses of the same type do not stack; therefore, Improved Initiative & Superior Initiative do not stack.
It is possible to retrain Improved Initiative for Superior Initiative once you reach epic tier (and it's a good idea to do so, since you're effectively getting +4 init for the price of a retrain), since the only requirement for Superior Initiative is that you be in the epic tier.