I've smoked marijuana a lot for the past 4 years but i want to get pregnant. would it be better to stop smoking? marijuana decrease chance of pregnancy?

Yes and yes: You should stop smoking anything - period.

Might decrease a bit: I believe marijuana can be a safe ; effective medicine ; it's ok to use recreationally on occasion, but i don't think it's healthy to use daily unless you have a clear medical need.It may slightly reduce chances of pregnancy though evidence is mixed. I don't think it's wise to use marijuana regularly during pregnancy due to potential harm to your baby, though evidence is mixed:http://bit.Ly/qdhtqu.