For Those who are still suffering from 4MB & 6 MB storage. I Came for your rescue. A friend of mine was having the same problem, a 6MB account, while I had a 100MB account. So I compared the differences and figured it out...
1. Login to your Yahoo! Mail account
2. If you see only 6MB or 4MB, go to Mail Options
3. Select Account Information from the left of the screen
4. You'll be asked for your password again. Enter it.
5. You will see the Membership Information page
6. Copy the url of this page into notepad
7. Compare your url with this url: http://edit.yahoo.com/config/set_intl?.child=&.scrumb=***********&.done=http%3a//edit.yahoo.com/config/eval_profile%3f.src=%26.intl=us%26.done=http%3a//us.f607.mail.yahoo.com/ym/Options%3fYY=87095%26.scrumb=***********
8. Copy your scrumb code (this should be the last 11 characters of your url) from your url and place it in the two places of my url where the *********** is. Copy this url (which has your scrumb code) back into your browser and press enter.
9. Change your settings to English - US
10. Accept the New User Agreement
11. You will be logged out
12. Go to mail.yahoo.com and relogin
13. You will now see a 100MB mailbox