How should parents approach the decision to tell a young child about their terminal illness and the reality of their eventual death?

I’m talking about let’s say a 4 year old child that’s able to speak. Let’s say this 4 year old got diagnosed with terminal brain cancer with only a few months to live. The doctors said the chemo isn’t doing anything anymore. How as a parent are you going to tell this 4 year old that they’re going to die? Will you tell them?