K.Z.
I know you say you want to nurse for 3-4 yrs, but try to change your thinking to say "until he is ready to quit." It could be 4yrs, but he could be done by 18 mos. You don't want to be stuck with a feeling that YOU are not done! This happened to a friend, she was ready to nurse til 5 and her son was done by 18 mos. Just throwing that out there. My son just weaned at 4.5yrs and having the attitude that it's available as long as he needs it was critical.
From early on mine was a furious nurser. Just wanted it all the time. He seems to need human touch for comfort and now we still snuggle a lot, he likes to be held.
Just know your son. At a certain point (12-18 mos, when they're walking and talking), you don't offer but also don't refuse. Let them ask for it and when they do, let them have it! When it is available, they get to decide when they need it and when they don't need it anymore and weaning is so natural. And yes, it does happen!
It can get rough in there too! Reach out for support, read books, talk to people you know who did extended nursing! We are out there. ;)