Hi
I think you can do what you want using s_4_back_price etc. and it would be safer to just avoid betting in markets if there is a withdrawn runner or reserve. Betfair rules are clear bets are on trap numbers, not selection names.
To prevent betting in these markets, if that is what you prefer, you can have a number of safety conditions:
withdrawn_number is equal to 0
runner_number is equal to 6
every selections name does not contain "(Res)"