Breast: Preschooler

How to Explain Death to a 4 Year Old???

Here's the thing... my Father in law was diagnosed with Cancer about a month and a half ago, its been quick, and we dont think he's got much time left... my 4 year old sees him all the time, he knows that he's not feeling well, since he's been weak... what do i do? Do i prepare him for it? do i just talk to him afterwards? what do i say??? i dont want him to be afraid, or afraid of being sick... has anyone dealt with this? Any advise welcome...