Just create 8 bars of music in subproject, but insert those markers at e.g bar 3 and 5.
then main project will request re-rendering of subproject and only this part will be rendered. If you move markers to different part in subproject, e.g. bar 7 and 9, then different 2 bar piece will reappear in main project
Try it for yourself as I cannot verify right now but played with this some time ago.
the markers mus be named exactly this way:
=START
=END
This defines exactly which part of the project is "active".
EDIT: I just tried it again here and confirming that it works like that flawlessly.