Why did Dustin Diamond's stage 4 cancer kill him faster than Shannon Doherty?

Because it was caught late.

She caught hers much earlier. 

Dustin Diamond's cancer was caught late, and he died less than a month after it was diagnosed. Shannen Doherty has breast cancer, and not all cancers are the same so there may or may not be a chance that she might make it to the end of the year.

Shannen Doherty's last face lift scared her cancer away.

Because it was caught late.

I don't believe her's was at stage 4.

May be she had her's caught  in time to save her, but her acting. 

She was a ***** to work with.

There are 2 ways cancer is measured. One is stage (1 - 4) which is a measurement of how much the cancer has spread through the body at the time of diagnosis. The other is grade (1 - 4) which has to do with how aggressive this particular cancer is. Mr. Diamond could have had a higher grade cancer than Ms. Doherty. 

There are other factors too. One person might have been stronger/ healthier before the cancer, and had more strength to fight the cancer's effect on their body. Or their might be a difference in the treatment plan, with one method being more effective than another. Even mental health factors can influence how a person fares against cancer. Someone with a strong spirit and positive outlook often has an edge over someone with depression. Finally, it makes a difference where in the body the cancer has spread to. Cancer that has moved into the lungs or the brain or bone marrow presents more challenges than cancer somewhere else in the body. 

Different types of cancer develop at different rates.

We do not know how long he had the cancer before it announced it.

We do not how far advanced it was before they discovered it

We do not know what treatments he underwent

We do not know if he was doing things that might have continued to advance the cancer while Doherty is not

Not enough details to know why he went so fast.

Shannon Doherty wasn't killing him.

His spread much faster.

Also he was diagnosed when it was too late unfortunately, didn’t catch when it was in early stages. He waited too long to get checked out