I may never sell it and do NOT plan on getting an iPhone 5.
First off, my iPhone 4 works great for my needs. It is hard to keep up with technology. You never know if we upgrade like in our apps that it means it will improve. Sometimes the hardware is still unproven. My iPhone 4 is the most stable phone I have ever had and don't want to lose that for something that looks to be better on paper. It is special to me anyway because I got it on launch day which was a smooth experience and #4 is one of my favorite numbers because my birthday falls on that number.
Secondly, I'm getting bored of iOS already and may want to try something different. I went through the jailbreaking scene with all the customizations and whatnots. Had Intelliscreen, vWallpaper, Customize, Live Clock icon, Live Weather icon, SBSettings (Boss Prefs), and even made it look like those Vista themes using Folders and Poof during those early days when the App Store was still immature and useless.
Since I like to carry two different phones (one w/ contract & one prepaid), I may go Android. I am really looking closely at the XPERIA Play again and curious what type of gaming it brings. I like to try different things to expand my horizons like in music genres. Even in video game consoles, I end up buying them all because they all bring certain things to the table and never get myself into insecure fanboy arguments. In case someone has a problem with an OS, I can always help them out. Keep myself versatile with the different OSes and expand my knowledge and not simply with iPhone. It doesn't mean I will sell my iPhone 4 if I go Android. I can always demote it as a back-up phone/iPod touch when my contract is up. I have too much investments with my apps that puts me locked in a ball & chain with Apple.