At four weeks, you're probably still feeling the side effects of Zoloft. I have been taking it at a very low dose for a few weeks now, and I am still having trouble sleeping. But I've been on Zoloft before, and I know it will get better. I recommend that you keep taking it for at least a couple more weeks and see if it gets better for you, too. Good luck!
Try taking the Seroquel for just a few nights. You can do better if you just get your rest caught up.