Yes if you clear by week 4 you still need to finish treatment being undetectible is only a part of the battle unfortunately.
Definitely use condoms - we are advised to use two types of birth control when we are on treatment, the risk of birth defects is that real.
You shouldn't get pregnant or get someone pregnant while on tx or in the 6 months following tx. As long as your girlfriend doesn't handle your ribavirin tablets and you use condoms, I don't see where you doing tx should be a problem.
Keep in mind that even if you test undetected on your 4 week viral load test, you will still need to continue tx.