I have an Oyster card with a Zone1-3 annual travelcard and something like
£15 of prepay on it

If I want to travel from Earlsfield (zone3) to Kew (also zone 3), going via
Richmond (zone 4), I only ever touch in and out in zone 3 so I am not
paying for my brief time in zone 4. However I have done what is expected of
me - touched in and out and carried enough prepay to cover my extension
ticket.

A colleague tells me that this is correct and that I am travelling perfectly
legally - can anyone here confirm this?

Thanks
Tim