How to downgrade iPhone 4 from 4.0.2 to 4.0.1?

[torbengb]

Here's what happened:

1. My iPhone 4 shipped with iOS 4.0.1.
2. I went to JailbreakMe and slid the slider. (WOW, easiest jailbreak ever!)
3. The jailbreak did its thing and rebooted the phone.
4. The phone was stuck in boot! It never got past the silver-on-black Apple logo. I first waited 20 minutes, then did a hard-reset and waited again (a few times), but it obviously got stuck somewhere in the boot process.
5. I really need the phone, so I put it in DFU mode and restored from iTunes ... but by doing that, it was upgraded to 4.0.2!

Question: How can I downgrade my iPhone 4 from 4.0.2 to 4.0.1?

[Simtech]

If you don't have your 4.0.1 SHSH files stored, you can't.

[torbengb]

As I understand it, Cydia offers to back up this SHSH file after the jailbreak; and before the jailbreak I can't get to the file because, well, it's not jailbroken yet. Since my phone couldn't boot after jailbreak, I didn't have a chance to let Cydia make a backup.

Is this file specific to my phone, or can I get the SHSH file from somewhere else?

[Simtech]

You can get your files saved prior to jailbreaking with the umbrella utility. I had my SHSH files saved several weeks before the jailbreak was released. And No, SHSH files are unique to each phone.

[torbengb]

This is very useful to know; I wish I had known before. (It would have been nice if the jailbreak website made a mention of that, but on the other hand it's not their concern.)

[Simtech]

This site as well as many others stated to get your SHSH files as soon as possible via Umbrella.

[torbengb]

Thanks; I've learned the lesson :-) This won't happen to me again!

Finding info about Umbrella took some googling; it wasn't easily found here on modmyi. So as a helping hand for other readers who come across this thread, here's the story: (my highlights)

Apple added an extra layer of security [...] If you accidentally upgrade your jailbroken device to Apple’s latest firmware, you can’t re-jailbreak it until hackers release new jailbreak software. You can, however, downgrade your device to the previous firmware version which can be jailbroken — if you have your SHSH blobs on file. You can extract these SHSH records and save them with the help of a utility called Umbrella.

Umbrella for Windows is described here, with download links:
iClarified - iPhone - How to Backup Your SHSH Blobs Using Firmware Umbrella [Windows]

Umbrella for Mac is described here, with download links:
Before Jailbreaking, Extract Your iPhone’s SHSH Blobs with Umbrella [Jailbreak Superguide] | Cult of Mac