why does my iphone 4 kick me out of apps

Double-click your home button, and slide the app screen up to delete them--sounds like your RAM is low. If that doesn't work, try restarting your iPhone by holding the lock and home buttons until the Apple logo appears.

Probably out of available RAM. Go to the Home screen. Double-press the HOME button. Flick the screen image of every app up to kill the app. When they are all gone reset the phone by holding the HOME and SLEEP buttons at the same time until an Apple logo appears (10-15 seconds). You will not lose any data. Once the phone reboots the problem should be gone.

Isn't it nice when you get 2 answers and they agree? 😉

Great minds think alike, Lawrence. 😉

I like your style.

Each app takes up RAM. If you have too many apps open, then the RAM will fill up, and start kicking you out of apps. Remove each app after you use it if you do not need it any longer.

Try downloading iMovie from iTunes on your computer, then syncing it to your device.

What Finest said. Normally iOS will first move apps in memory to make a block big enough for the app you chose, then delete the oldest app to free up memory when it is needed if that is not sufficient, but sometimes that app has a background process running and can't be deleted. This causes a deadlock situation and the new app fails to load. So this should occur only rarely. But once it happens you need to kill all apps so RAM allocation can start over.

Maybe this will help, sealteamtango: http://support.apple.com/kb/HT1725.