In ups.com,
Andrea typed:

My local underground station (Holland Park) didn't have the info to
hand, so can somebody kindly tell me which National Rail/tram routes
in London my annual Z1-3 Oystercard can be used on?

All National Rail routes which fall within your zones.
All of Tramlink



Is it also true that I can use any bus service in greater London,
despite only being able to go as far as Z3 on the tube?

Yes.



If I wanted to go to a station in Zone 4, do I have to buy a single
zone extension prior to travel? How much does that cost, e.g. Z3 to
Z4, or Z2 to Z3?

Yes, you would need to purchase the extension prior to travel. I'm not
sure how much it would be, but if the Z4 station that you want to travel
to is an Underground station, it would be cheaper for you to add some
Pre-Pay to you Oystercard and use that.





--
Bob