Like I already mentioned for obvious reasons they wont go with MT4, if they ever decide to offer MetaTrader it will be MT5 because only MT5 can be fitted to their backend. Mt4 for example can neither place nor display an existing pending order of the type they have at Oanda. This is only possible with MT5.
MT4 and the FXTrade platform are mutually exclusive, they could never be used to trade the same account, they would have to make MT4-only accounts.
MT5 will soon be the current release of MetaTrader and MT4 will be deprecated. They already stopped development for MT4, they don't even fix bugs anymore! One reason for this is MT4 is incompatible with the NFA rules from the ground up and unfortunately these rules are now undeniable part of our reality. MT4 cannot be fixed or adopted, it can only be completely replaced.
I doubt that Oanda will invest any money into trying to ride an already dead horse.