What are the reasons the check engine light would come on in a 2000 4-Runner?

There are way too many to list, probably a couple of thousand different things that could trigger it. When the light goes on, the engine computer stores a fault code. This is just a series of numbers or letters and numbers. You need to retrieve this code from the engine computer. Some autozone's have the equipment to diagnose a check engine light, and will do it for free if they can. Once you have the code, you can find out why the computer triggered the light, for example it may say "lean mixture", "knock", "cylinder misfire" etc. This will help you narrow down the possible causes from a few thousand to about a half a dozen things to check. If you don't understand what the fault code means, have a friend who knows a lot about cars help you. If you still can't get the problem solved, then you may have to take it to a mechanic and be satisfied that at least you tried to fix it yourself. Also, if you do have it fixed by a mechanic, be sure to ask them what caused it, most mechanics are very friendly about explaining what was wrong. That way, if in the future you get the same code, you'll know what to check first. Hope this helps. Good luck.