then how sure your gf is pregnant? cos you have result from 驗棒, didn't you bring your gf to see a doctor?
but i can say that even pregnant for 4 months, the body has not much change in size, provide that your gf eats a lot of good everyday, like double or triple on your usual meal size, so don't worry, in the coming 5th or 6th month, the baby will become larger and larger.
take it easy! but if you want more professional guidelines, you can go to
http://www.famplan.org.hk/fpahk/zh/template1.asp?style=template1.asp&content=home/mainpage.asp