My DD napped until she was a bit over 4. She still used a pacifier in bed, and we had told her that after she turned 4, she would have to give it up. Once she didn't have her pacifier anymore, she stopped napping. She really didn't need a nap much by then anyway!
My son started napping less frequently and skipping naps sometimes when he was around 3 1/2. I always made him lay down and rest, especially on preschool days. If he didn't have at least a break from constant play and activity, he was really a nightmare by the evening. If he even had a 45-minute rest period in his room without falling asleep, he'd usually be fine. Toward the end of the school year, we agreed that he could skip naps on weekends, but that he would still try to take a nap during the week. Once summer vacation started, he could skip naps altogether. That plan worked pretty well and got him ready for afternoon preschool that fall.