Call one sub-district N and the other S, the bracket would look like this:
N-1st
S-4th
N-3rd
S-2nd
N-2nd
S-3rd
N-4th
S-1st
The buys would fall in their normal spots. If the N sub only had two representatives then the N-3rd and N-4th slots would be buys.
Picking up on another thread…one reason that the 5th and 6th place matches were wrestled at sub-districts was to fill out these brackets. In this case, as allowed by some districts, the buys could be filled by the 5th and or 6th place wrestler(s) from the competing sub-district. I think this is a good policy, as it allows the kids another opportunity to compete.
If you wrestle all of your five man divisions in round robin format, be prepared to extend the length of your day. Doing the math there are 20 matches in a five man round robin and only 6 matches in eight man bracket with three buys.