My DS was also sleeping through the night around 7 or 8 weeks (but we always did dream feed and still do - he's 6 months old now). But a few weeks after I started 4 hour EASY - when DS was around 3 1/2 mo - my milk supply started to decrease. When I started 4 hour EASY, I also went down to 5 feedings a day (including df). I was able to bring my supply back up by pumping after feedings and taking herbs. But I've noticed dips in my supply a couple subsequent times and DS actually lost a little bit of weight at his 5 month checkup. I was doing an 11:30 df and the supply still dropped. I should also add that I was in crazy OVER supply when DS was a few months old, so I never had this issue early on. So I think you need to be extra careful if you started out with an inadequate supply. My LC said that it's always easier for oversuppliers to boost their supply later on and I've found that to be the case (it usually takes me 2 days to boost my supply).
And I absolutely agree with the previous poster about how important it is for night feedings to continue in order to maintain your milk supply. I love the BW book and method and wouldn't have done a thing differently. But I do think that using Tracy's methods to train a young baby to sleep through the night (as I did) can sometimes compromise milk supply later on. Prolactin levels are highest at night, so if the breasts are not stimulated at night, they start producing a lot less milk. I really love my sleep though, so I was never keen on waking in the middle of the night to pump
I hope this helps. You could always add another nursing session during the day if you find your supply is diminishing. You might also consider finding a way to weigh your baby every week while you make the transition, just to ensure weight gain. I live in New York City and a lot of the breastfeeding stores here have baby scales. Breastfeeding support groups would also have baby scales.
Good luck!