You're talking about an upper terminus cleanout for the 4" main? 4" pipe won't fit in a 2x4 wall, but it would fit in a 2x6 wall. If you wanted access via a floor plug, you'd just turn up the horizontal main with a LT90.
For wall access, you'd turn it up into the 2x6 wall; then I'm not sure what would be best within the wall. You could obviously use a large enough access panel, and just a cleanout plug at the top of the vertical, but maybe there an option that's better looking or provides better access.
Cheers, Wayne