I had a flawless install on my 4. I will say this I used my macbook pro to download the file first then I installed the program. I did that because it was on release day and thought that would be the smartest move for me.
I have only had a few issues with maps, 1 being it doesn't talk to me, the other is Siri wont talk to me. Of course both of those things are to be expected because the 4 wasn't going to get those features.
I have an iPod touch 4th gen which came out not last year(they only added the white faceplate) but the year before and while I have some annoyances with email overall performance is better than I expected for a single core device. It runs as zippy as before, except for the app store is a bit problematic, but I have heard of others having issues with that. Personally I would look up the features you'll get for the iPhone 4 and see if you want the update based on the features. But I don't think you will end up with a near broken phone if you update to iOS 6, so if you want I say go for it.
Downloaded iOS6 and I had two issue arise. First, I could not connect to wifi. It would redirect me to an Apple login page. The next day it worked fine. Second, none of the pictures I have assigned to contacts appear when that person is calling me. I don't have a clue how to fix this. Any suggestions would be great.