It is true that only the inboard engines have reverse thrust -- the outboard engines do not in order to reduce FOD (debris stirred up on landing). But the reverse thrust mechanism is electrically activated, and maybe for part commonality all engines have thrust reversers but it is not connected on the outboard engines? That would allow swapping inboard/outboard. I know the original A380 design had no reverse thrust at all, and this was modified (at the suggestion of the FAA) to add reverse thrust only to the inboard engines. Perhaps the final design absorbed the extra component weight for the sake of reliability and commonality.

As an aside, while all airliners are required to be certified for landing without using reverse thrust (brakes only), QF had a rather famous incident not far from SIN that was due to corporate policy for 747 pilots not to use reverse thrust on landing (to save engine wear/cycles). That repair was rumoured to cost more than the aircraft value (done by QF to keep their record intact of never having a jet hull loss in a crash). So QF is probably a reverse thrust fan these days.