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.
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.