Q: Where do I find my apps for my iPhone 4 on my computer? Is there a folder within Windows?
A: iTunes stores iOS applications in a folder named “Mobile Applications” that will either be stored directly under your iTunes folder within the “My Music” folder, or in your “iTunes Media” folder if you’ve converted to the new iTunes 9/10 file and folder organization. All iOS applications are named with an IPA extension, so you can also search for them this way.
Note that you shouldn’t normally need to worry too much about where your apps are stored on your computer, as iTunes normally manages this for you and there’s little reason to look at the underlying application packages unless you’re a more advanced user or developer. If your concern is backing them up, then simply ensuring that you’re backing up your main “iTunes” and “iTunes Media” folders should ensure that your apps are also included.