I was an Android user for a while, and was on Sprint with the Evo. I, too, had an iPod touch and felt I was already familiar with the OS and wondered the same things..... is it any different? My answer to that is yes. The ipod touch is *basically* an iphone, without the phone..... but you are really limited for connectivity. Having an iphone opens up a whole new world of possibilities coming from the ipod touch. You have a whole new world of apps that you can use by having access to a cellular network and being connected all the time. I found myself using my ipod touch for music only, because everything else of interest needed a data connection to be useful, which I did not always have, not to mention GPS etc.
I couldn't wait for the iphone 5, I think it is kinda silly to sit around always waiting for the next big thing, because by the time it is released, there is always something better just around the corner. While the iphone 4 might be last year's technology, it is certainly not going to be obsolete any time soon. I'm just glad that I was able to hop on the band wagon with the iphone 4 instead of being stuck with a 3GS, the front facing camera is a pretty big deal to me, as well as the battery life.
Coming from Android, there are a lot of quirks that drive me nuts, and I am sure you will get frustrated at first as well, but after a week or two, I have really started to enjoy the iphone. For the record, I really really like Android, but feel that the hardware just has not caught up with the software. The OS is awesome, but the handsets are really sub-par IMO compared to the iphone. Apple has really nailed it when it comes to hardware design with the iphone, and battery life is superb compared to the Android handsets available. It was a tough decision for me too, but I am glad I got the iphone, and though I will be excited to see what iOS 5 brings to the table, I will pass on the iphone 5, and be ready for an upgrade when the iphone 6 comes around.
Good luck!