Is it safe to factory restore my jailbroken iPhone 4 - fw 4.2.1, greenpoison rc6

[Endymion86]

A loooooooong time ago, a friend of mine and I jailbroke my iPhone 4 using greenpoison rc6 (or rc 5... I honetly can't remember which, and don't know how to find out now).

I've read conflicting stories on various sites about whether or not it's safe to do a factory restore in iTunes. I'd like to just restore and update the phone to the latest official firmware, in preparation for iOS5.

So what I'm asking is... do any of you know if it's safe to try to restore using iTunes? Will it hang/brick my phone? These may sound like dumb questions, but I only know enough about all of this to be dangerous; I'm not an expert by any means.

I'll provide any other info you guys need.

ATT iPhone 4, jailbroken firmware 4.2.1 (8C148), greenpoison rc6 (or rc5... I honestly can't remember which, and don't know how to check now).

Thanks in advance!

[Simon]

You can restore to 4.3.5 no problem but if you want to jailbreak again there is only a tethered jailbreak for 4.3.5. You'd be better off just staying put until a untethered jailbreak is released for iOS 5 sometime after it comes out.

[Endymion86]

Awesome, thanks man. I'm assuming 4.3.5 is the latest fw available on iTunes at the moment? I.e., could I factory restore, then update my fw officially through iTunes, and it'd be on 4.3.5?

[Simon]

iTunes will force you to 4.3.5 if you try to restore.

[Endymion86]

Perfect, thanks!