Diagram what would happen if sexual reproduction took place for four generations using diploid (2n) cells.

It wouldn't. Zygotes that have more than the standard chromosome number do not develop; in fact, in animals, trisomy (inheriting 3 chromosomes instead of the normal two) is fatal. The exception for this is trisomy-21, or Down's Syndrome. 

Hypothjetically, let's say it worked and a diploid zygote was fertilized with another diploid (2n) zygote...for 4 generations. In humans, that would result in a zygote containing 186 chromosomes instead of the human chromosome number of 46 (2n). 

I guess you could say it would have a chromosome number of 4n.