how can you unlock an iphone 4 after is has been disabled?

Once the Device is asking to be Restored with iTunes... it is too late to save anything... and you must continue with the Recovery...

See Here > http://support.apple.com/kb/HT1808

You may need to try this More than Once... Be sure to Follow ALL the Steps...

After you have Recovered your Device...

Re-Sync your Content or Restore from the most recent Backup...

Restore from Backup > http://support.apple.com/kb/ht1766

-> http://support.apple.com/kb/ht1212