Thanks for the question. You can definitely purchase Tones from the iTunes Store on your iPhone 4. Here's how to do so:
You cannot purchase ringtones and alert tones from your computer, but you can purchase them using your iOS device with iOS 5 or later. You can purchase tones on iPad, iPhone (3GS or later), and iPod touch (4th generation or later).
To locate tones in iTunes on an iOS device:
1. Tap the iTunes app.
2. Tap Tones. If Tones is not visible, tap More > Tones.
3. Browse for tones. Once you purchase a tone, a dialog is displayed. The dialog gives you an option to set your new tone as your default tone or assign it to a Contact. If you don't want to do anything with the tone just yet, simply tap Done.
Tones purchased using your iOS device are synced to your iTunes library when you connect your device to your computer. On an iPod touch or iPhone, you can find tones in Settings > Sounds. On an iPad, you can find tones in Settings > General > Sounds.
Note: General music that is purchased from the iTunes Store cannot be set as a ringtone, you can purcahse a separate Tone from the iTunes Store or explore third-party applications offered in the App Store.