Restore iPhone from a jailbroken backup with old firmware?

Hi there,

I have a iPhone 3G that was running on 3.1.2, jailbroken with blackra1n and unlocked. But it crashed today and I had to restore it and it got upgraded to 4.2.1.

But fortunately, I have a backup and therefore I wonder if I can restore my iPhone that runs on iOS4 now, with my old backup that is from iOS3 and jailbroken and unlocked?

What does the backup actually contain? Is it just sms, contacts photos, apps etc. or also jailbreak and unlock files?

Is it going to harm my phone if I proceed with the restore?

All answers are appreciated!

 

Thank you for your quick reply! If I jailbreak and unlock my iPhone first, and then do the restore from my backup, will the jailbreak and unlock disappear?