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?

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?

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?

Thanks