Our contractor's Electrician has just placed a 4 outlet metal box directly under the sink waste trap... We are questioning if this is safe... Is it safe??

I assume you mean in a wall adjacent to or in close proximity to the trap. I also assume you mean in a kitchen cabinet. I can't think of any reason to put an outlet in a bath cabinet.

If my assumptions are correct, no problem. These would be for connection of disposal, possibly dishwasher (many now come cord and plug) and something else like instant hot water (Hot Shot).

I wouldn't want an outlet in the floor of my cabinet.