If I convert my app to ARC, will it still work on devices running iOS 4?

I just converted my app to ARC, will the update 开发者_高级运维still work on devices that are running iOS 4?

Also, what about the new UIAlertView Text input styles?

ARC code runs on iOS 4.0+ and Mac OS X 10.6+, however, you can't use the zeroing-weak references. That's only supported on iOS 5+ and Mac OS X 10.7+.

This is documented in Transitioning to ARC Release Notes.