so there's no real risk to jailbreaking my iphone 4, right? i can just back it up first, and then restore if i need to?
There's absolutely no risk.
If something goes wrong, you can always get into recovery mode and restore.
I've had my iPod Touch 2G jailbroken nearly ever since I've had it. I was jailbroken on 2.2.1 for over two years, and it worked just fine until I tried overclocking and it didn't work out very well. Then, I went to 4.0 and used that from August 2010 to a month ago, when I retired my iPod 'cuz I got a Droid 2 Global, which is infinitely better.