Can't downgrade iPhone 4 ios 4.0

I have the first gen iphone 4 with ios 5.9.1. The ios 5.0 is slow & buggy. I want to downgrade to ios 4.0. But it just won't install on my iphone.

No, it won't you're correct.

v4 is no longer signed by Apple.

In order to downgrade you need SHSH blobs from the version you want to downgrade to, the IPSW file for that version and you need to be prepared to play around with your hosts file to stop iTunes dialing home for the downgrade.

I've not heard of, or experienced (I have 2 iPads, 2 iPhone 4 and an iPhone 4s all on iOS 5.1.1 in the household) any significant iOS5 bugs. Certainly not enough to warrant ditching the features of 5 to go back to 4.

What issues are you having in particular?

Have you tried a hard reset - backup - sync - restore - sync ?

iOS 5 is very stable on my iPhone 4, as mrplow suggested, you might want to try a restore first before trying to downgrade to iOS 4..

No idea what you mean by "iOS 5 is slow and buggy." That's not the experience of the millions who have bought the iPhone of late (and the millions more who've upgraded).

I'd say that problem is likely on your end, but you may also want to do a reset and reinstall and check with a tech if it continues being a poor experience for you.