The tests are pretty easy, but I did take them after a 2 year course at a local school. If you've been doing this for 4 years, just do some reading and take a few practice tests and you should be fine.
http://www.aa1car.com/
Lots of information here, sensors in particular.
http://freeasestudyguides.com/
A quick google search popped up with this. Didn't look into it much, but it seems pretty useful for practice questions.