lol the million dollar question...its possible but the 4 day course is only going to give you a taste. I did a week course and since finishing that about 6 months ago i've been solidly reading up on materials, substrates, hole cutting, different types of tile, laying different patterns, etc etc etc it goes on forever so much to learn and practice. But as a plasterer you will be fine, good setting out is often the key to good tiling. I doubt very much u'll earn a grand a week in domestic tiling...but certainly in commercial if you were doing large conservatory floors for examlple day in day out...but ure gonna have to be a a fast quality fixer to win those contracts.
defo a good skill to add but u'll soon find that u will need to be able to do basic plumbing also.