The last station in Zone 4 is Sydenham Norwood Junction, but you'd have to get a train which stops there, limiting choice severely. Also, trains may be diverted away from the normal route.
A Redhill - Zone 5/6 Season is £39.70 weekly, or £152.50 for a month. With your Z1-4 Oyster Season, this would allow travel on non-stop trains between Croydon and London Bridge (anywhere in Z1-6 in fact).
Redhill - Zones 1-6 is £71.10 weekly / £273.10 monthly.
Thus it would be slightly cheaper to buy a single Redhill-Zones 1-6 ticket for the duration you need to travel to Redhill. (A weekly Z1-4 Travelcard plus Redhill-Z5/6 comes to £79.10). What Travelcard do you hold currently - weekly, monthly, annual? I assume you live in London and thus need to travel within the Zones - if you only need London Bridge-Redhill then it's £58.60 weekly/£225.10 monthly.
Bear in mind Season tickets can be issued for any period from a month to a year, so if you know you'll be travelling for say 6 weeks, you can buy a ticket valid up to the last day you'll need it (and then revert to using an Oyster Travelcard).