wHY DOES MY CHECK CYLINDER #4 LIGHT COMES ON ON MY 2000 DODGE RAM 1500 VAN?

The computer is seeing a misfire on that cylinder. I would check ignition wire/s first then plugs. Not sure how many miles you have on the van, but if these are original plugs and wires its probably time for an ignition tune up. Another thing to look at is compression. This is a good diagnostic test and tells you much about the condition of your engine. You can also hook up a vacuum gauge to your engines intake vacuum and this will tell you a lot about your engine as well. Here is a good reference for vacuum trouble shooting.......... You should get between 17-22 in. hg. depending on your elevation above 2000 feet MSL or mean sea level. To perform this test hook vacuum gauge directly to manifold vacuum. 1. Low steady needle usually indicates leaky intake, a leaky vacuum hose, late timing, or incorrect camshaft timing. 2. If reading is 3-8 inches low and fluctuates at that low reading suspect an intake manifold gasket leak at intake port or faulty injector. 3. If the needle has regular drops of about 2-4 in hg. at a steady rate the valves are probably leaking. (Perform a compression check). 4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire. 5. A rapid vibration of about four in hg. indicates worn valve guides. 6. Slight fluctuation of one inch or so indicates ignition problems. 7. Large fluctuation perform a compression check. 8. If needle moves slowly through a wide range check for PCV issues. Hope this gives you a good place to start and helps you figure it out.