It's possible to set Reaper up for four point and three point editing. I started to work on custom Reaper 'actions' to do this and never finished it, but you can see what I was working on at Classical and Speech Editing Actions - Cockos Confederated Forums
on the Reaper forums.
For instance, one thing I had working was "four point editing with preview".
This places markers at the start and end of a section you wish to replace, and at the start and end of the material to be used as a replacement. The replacement material is copied, not moved, from the source location to the target location, replacing the designated area there. Before actually committing to the edit, it's previewed with a basic crossfade.
I haven't tried it with recent versions, nor on a Mac, but it does illustrate that if prepared to get your hands dirty, it's surprising what you can achieve with Reaper's custom actions etc.
This thread has reminded me of the whole subject and I've raised it again on the Reaper forums at Three point and four point editing etc - Cockos Confederated Forums
- whether the developers 'bite' remains to be seen.