If you can document exactly or close to how much time she has actually spent with the kids since the original order started, you might want to consider requesting a review of the support. Of course, this will probably piss her off royally, because I bet you are her only income. But if your state bases support on income and time spent, you should consider this. You may still have to pay her something (call it hidden alimony), but given the amount of time you have the kids, you have every right to ask for a reduction.
Because of her previous drug conviction, you have every right to demand that she submit to random drug testing and if she fails, she cannot exercise her visitation until she tests clean. I would also include (if possible) that if she tests positive over a certain number of times, then she loses all visitation until she tests clean for say 6 months. Or something comparable. Just make sure that there are safeguards in place for the kids (because of her drug use) and repercussions against her if she screws up.
If you've had the kids for 4 years, I agree with Giggles. Judges like to maintain 'status quo' and if your kids have done well with you (and he sees her history), he will not want to interupt that. One other thing you might want to have for documentation is reports from school showing how good they are doing and how stable they are.
But I wouldn't go into this refusing her everything. You have to be the bigger person and throw her some kind of bone. But I would also demand a stepped up visitation schedule, starting out small and if the kids can handle it, then gradually increasing the time. Start small and also include consequences if she fails to come through for the kids, like with the drug issue I mentioned above. Like basing the stepped up schedule based on how many times she sees the kids, NOT in months or weeks. That way, if she fails to show up for visitation, it will be just that much longer before it is increased, instead of not showing up numerous times and then having it increase, even though the kids haven't seen her much. Get what I mean?