Has iOS7 made your iPhone 4 excruciatingly slow?
Here is the setting you should switch on : Settings>General>Accessibility>Increase Contrast>On.
Besides this also make sure you have enough free space left by deleting apps, music, photos, videos you don’t need on your phone and close apps that run in the background by double clicking your home button and flicking unneeded apps up and of the screen. Leave your Phone and Messages on!