Yes. 4.2.x will be the line that compiles with the OMNET 5.x (or you can use 3.7.x if you need 3.x inet version fro some reason). If you are not comfortable using the 6.0 previews, you must stick with the 4.2 branch.
As we add features in lockstep between OMNET 6 previews and INET 4.3+ we are also backporting changes to the 3.7.x, 4.2.x and 4.3.x and inet master.
So in essence:
OMNET 5.x -> use INET 4.2 or 3.7
OMNET 6 -> use INET 4.3+ (or 3.7 if really necessary)
Obviously, there is a lot of additional functionality in INET 4.3+. compared to INET 4.2. (there is heavy work on TSN networking which will come in INET 4.4)
And to sow a bit more confusion, we *might* release OMNET 5.7 too around the final OMNET 6 release with some minor changes backported to ease the migration people.