just wondering if it is possible to revert back to ios 4 from ios 5. If possible then how?

Depends on the phone. If you have an old-bootrom 3GS or a 3G, it's fairly trivial to do. On the new bootrom iPhone 3GS or newer you need an SHSH for the target revision of iOS, and if you've already upgraded to iOS 6, it's extremely difficult and requires both an iOS 5.x SHSH and an iOS 4.x SHSH.

Short answer, if you don't know what an SHSH is check the link in my signature and check your Cydia in hopes that it cached some. If you have no SHSH and are on a new bootrom iPhone 3GS or newer, there is absolutely no way to downgrade the phone. To stave off the inevitable questions:

1) No one else's SHSH will work for your phone, unfortunately.
2) You can't get an SHSH for the version of iOS that you want it for unless Apple is currently signing it. You can check the iPhone Wiki to see what versions are currently being signed.
3) I wasn't kidding when I said that there is no way around this that is known yet for devices newer than the old-bootrom iPhone 3GS. As there is no bootrom exploit, there is no way to bypass the SHSH requirement. Since iOS 5.0+ includes the random apticket nonce as well as other protections, it is much more difficult to downgrade once you have upgraded to iOS 5 or newer.

I saw in another post you mentioned you had a 3GS. If that is the phone you're trying to downgrade, find out whether it is a new bootrom or old bootrom. Old bootroms are easy to downgrade since there is a low level hack, new bootrom phones are much more difficult and can brick more easily.

I know you asked about going down to iOS 4 from iOS 5, but the instructions below should suffice for a new bootrom 3GS, just replace the ipsw files and SHSH files with your respective version.

http://www.iclarified.com/25227/how-...ios-5x-windows

If you have an old bootrom device, it's a lot easier. Download redsn0w or sn0wbreeze and make a custom firmware for the target revision, put the phone into DFU or pwned DFU mode (can't remember which you'd need, it will say in the app) and restore.