If I understood you correctly, the picture you showed actually works how you want, but you wanted to put the 1st track as a parent track, and the other 3 as children?
One thing to note on that... I believe if you route from the parent to the children, then the children back to the parent, you create a feedback loop. Reaper, by default, will prevent this and shut off audio on the tracks (you can override this in settings, if this is intentional, but in this case, it's clearly not). So make sure you aren't doing that.
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