Basically, if you use any of the new features that were introduced in Dorico 4 that result in something new appearing in the music that didn’t appear before – things like numbered bar regions, chord diagram fingerings, capo chord symbols, etc. etc. – then those things will not appear in Dorico 3.5, and when the project is re-saved in Dorico 3.5, those items will be deleted (because Dorico 3.5 doesn’t know about them).
There is also no guarantee that the music will be spaced and laid out precisely the same, but provided you’re not planning on doing the final engraving work in Dorico 4 and then outputting from Dorico 3.5, that shouldn’t be a problem.
Obviously we would recommend that if you are collaborating closely with somebody on a project, it would be optimal for both of you to be using the same version of the software.