Try while iOS5.0.1 is still signed, so you can wipe and start over if you cannot.
It is most likely that you will get no problems, the only problems I could imagine is restoring the preference files, you might want to skip that to be safe (paranoid?) and set the preferences manually.
It should work. I've never used the contact backup method, just DropBox, but I've never had issues restoring pieces of a backup across different devices or iOS versions. I would just try it like the other person said. Worst case, you brick your phone but can still recover before 5.0.1 stops being signed.
I wouldn't.... only because some of the packages you had installed on your iPhone 4 may not be fully compatible with the 4s. Not that that fact is an issue itself if you can uninstall the offending package... but since pkgbkup installs everything at once, you may have a hard time trying to figure out which app is the one causing any issues.
I would recommend installing one by one from Cydia to make sure everything is stable and working before you add another. At least if something causes a crash then you know what did it!