How the iPhone 4 ages with iOS 7.
The iPhone 4 has the worst specs out of all the iOS devices that can run iOS 7, and there are many people who don't know if they should upgrade their own to it on the 18th. I just thought I'd like to share my experience to help you with your decision.
When using the device, the main thing you will notice is that there is some stutter when doing certain things, like switching between home screen pages, opening and closing folders, and opening and closing Notification Center and Control Center. The animations and everything take the same amount of time than with a higher spec'd phone, like the iPhone 5; it's just that the animations are not as smooth. It really depends what you are doing, though; in apps like Safari animations and scrolling are pretty damn smooth but the scrolling and such have more stutter in more graphical apps like Photos.
There are no dynamic wallpapers, so you won't get any motion responsive circles as your background. You can only set still photos. Additionally, the parallax effect has been removed. There's no letterpress style text in apps like Notes and Reminders. Many of the visual effects have been removed also.
Notification Center is almost pure black but there's a tiny bit of transparency, and there's no blur behind. Same with Control Center but it is gray instead of black. Toolbars and menus, like the title bar in Safari, are slightly transparent but there's no blur behind them.
The camera has some differences also. The Burst mode added in the GM does exist on the 4, so that's good. You can't apply filters while in the camera view but you can apply them on existing photos. You can't zoom while taking video, which kind of sucks. But the camera hasn't really aged and is still as good as it was in 2010.
However, there are still great new features. Despite many visual effects being removed, the functionality still exists. Voice Control has been redesigned to look a bit more like Siri with the list of things you can say and the voice wave at the bottom. Unlike other iOS releases, the performance actually has been IMPROVED a little on some apps. For example, the App Store loads very fast, at least 3 times faster than on iOS 6.
I could go on and on, but I think you have better things to do than read a wall of text. Go ahead and comment if you have any questions and I'll try to answer them.
I personally would recommend upgrading unless you want to keep your jailbreak or something. There are many improvements and features and the new design is great. They all make up for the slight performance hit with certain things.