Be careful here and Darren is very correct. If you open a V3 song in V4 and save in V4, you need to change the title at that point. I usually add V4 into the title. Then your V3 song will always be intact.

As Darren also says mostly no problem but there can be issues. When they went from V2 to V3 (or even 3 to 3.5 can't remember which) they made changes to the way compressors worked and sessions did sound different. If you feel for any reason your V3 sessions are not sounding the same, it is best to finish them off completely in V3. The developers have often made that very clear. They say it is always best to start new fresh projects in the later version.

There is no big thing in V4 that is suddenly going to make your music sound way better. Unless of course you want to start changing chord progressions or add in patterns etc..