Not a heavy pilot but I would have thought that inboard engines in reverse thrust would be more likely to throw stuff into the outboard than vice versa. If there is a structural or aerodynamic reason for inboard only I would think it would have to do with the lateral moments of one outer engine failing to deploy.
Unlike #2 and #3, the outboard engines will not be over tarmac (or very nearthe edge) at many airports during rollout - hence the increased chance of FOD from the unpaved area. With regard to FOD due to reversers, the engine ingests it's own wash.