When last I installed SQL Developer, it was not an install, but rather just unzipping and then running the executable. I installed SQL Developer 4 while SQL Developer 3 was already on the PC. No issues.
get version 4.1...or wait a bit and get version 4.2 - when you DO get it...
- extract to a NEW directory, something like c:\oracle\sqldev\4.2
- run it
- say 'yes' to copying over your old stuff
- update the windows shortcut to point to the EXE in the 4.2\sqldeveloper directory
- delete the old sqldev folder if you need to save space