As previously stated by another poster i think the most likely reason for selecting to keep the reversers on the inboard engines is to minimise lateral moments should one reverser fail.[/QUOTE]
This is a valid point. From experience on the B707 and DC 8 we normally used only the inboard reversers. However we used to deploy all the four reversers but used idle reverse on the outboards and up to full reverse on the inboards. The reason for deploying all four was that in case there was an inboard reverser failure and stopping became a problem then full reverse was available on the outboards.