As of 2015,
You can renew your F4 visa only 2 months before it ends. So you can't renew it 6 months before your visa expires.
Also to change your address, you just have to go to your nearest "gu-chung" (city or district hall) before 14 days after moving.
Toll free number is 1345.
You can get to a rather friendly voice with different languages.
Hope that helps anyone.