Secret trick with the editor to use 4 and more time bars...

[nimbleswitch]

Great tip! And It works with D.S. al 3rd, etc., too, I see. And both work in iOS, too. iRP's Playback can't be responding to the actual bracket numbers above 3, I guess, but it does appear that it treats every initially unnumbered ending bracket as a "Next" bracket, which is just as good.
I just now used it all the way to a bracket no. 9 to make a backing track for a well-known Disney film toon. Thanks!

EDIT: An odd quirk: This trick with D.S. does not work if you have used an ordinary repeat section with ordinary 1 & 2 numbered repeat brackets previously in the song. Take either one out and the other works fine, but not both in the same song. Go figure.