I think you should encourage him to wean on his birthday and really talk it up about how exciting it is going to be because he is weaning and turning four. If you are so tired of nursing that you can't do it for more than the time it takes to sing the ABC song then you really need to find a way to be done before you start to resent him. You might consider still letting him nurse but maybe just one or two times a day and then cutting back on those if he isn't able to stick with it though, but if you talk it up he may be really eager to actually wean and excited about weaning.
Also, keep him busy during the day by going off to do activities outside. It may be he doesn't get active because he doesn't have a chance to so try to find things to keep him active and do them with him. If he is nursing just out of boredom because you don't do other things together then find other things to do even if it is just playing clay, reading, talking, drawing, or taking a short walk.