If I start boxing Amateur again now, could I have a chance at Olympics in 4 years?

You could go from zero experience to Olympics in 4 years (Joshua did). It's all down to how much you put into it and how talented you are. That said, the higher your weight class the better your chances. 178 and up is really anybody's game from what I've seen.

I will say that you'd probably be the first Olympian that had to ask teh interwebz whether he had a chance or not. I mean, if you have to ask...