Need to know how to downgrade from iOS 4.0.2 to iOS 4.0.1 without saving the SHSH Blobs? Below is an easy-to-read and compact guide aimed at this.      

[How To] Downgrade From iOS 4.0.2 to iOS 4.0.1 :

1) Firstly, download the iOS 4.0.1 firmware.

2) Now, extract it with winrar/winzip  to a folder. You may need to rename the firmware file from .ipsw to .zip to extract.

3) After extraction, open the "buildmanifest.plist" with notepad. Using "Ctrl+F" and replace all – 8A306 with 8A400. Save the file and repeat the same with the file "restore.plist".

4) Now download iOS 4.0.2 firmware and open it with winrar/winzip.

Note: Do not extract it. Just open it and leave it as it is. Use this file only and don't create a new one. If you need to create a new file if you are on OS X, then use zip command line to make the zip. We assume that you are using the original file opened in winrar.

5) Once you have iOS 4.0.2 file open in winrar, take all the files from the 4.0.1 and drag them over to the
4.0.2 zip archive that you have opened.

6) Delete all the dmg files that have 002 at the end, leaving only the 001 files left.

7) Save the archive and rename it back to .ipsw if you changed the name to get winrar/winzip to open it.

8) Once you have renamed your firmware file back to .ipsw file, you have to add 74.208.10.249
gs.apple.com line to the hosts file.

9) Now put your iDevice into dfu mode, and open iTunes to restore the firmware you just created.

10) Done.

As always, do let us know how it goes.