Postgres CYCLE and existing values
The sequence will return values in order, whether or not they exist in the table. Your code will need to defend against that. Any method to defend against possibly already-in-use serial values will really hurt concurrency - and if your DB is big enough that you're wrapping int4 serial columns, you care about concurrency.

Most people just use bigint keys and don't worry about ID re-use.  You won't run out.