Typically, when the ios upgrades the developers of the apps are forced to upgrade their app software to be compatible with the newest version.
So you won't find any apps in the app store that are compatible with ios 4.3
You can check what software the app needs in the app store under the picture of the app and the price
It will say category and under that will be a title that says requirements.
That will tell you if any apps are compatible with your iphone. Most likely you will not find any.
It doesn't mean you don't have a good iphone though. It's still a great mp3 player and camera as well as a decent internet surfing device. If you're fine without the apps you can still enjoy it.