Sync your iPhone 4 with Yahoo services so you could use your email account directly on your iphone 4.
1. Go to the Settings screen.
2. Tap on Mail, Contacts, Calendars.
3. Next tap on Add account.
4. Then tap on Yahoo.
5. Now enter in the require fields such as name, username, email, password and description.
6. Finally you can choose the available services to synchronise to your iPhone 4.
You could now view and send Yahoo email on your iPhone 4.