I don't have children, but I'm a carer for my parents, and I had to think how and when to tell them. I decided I could hide it well for a while, so I waited until I had the full diagnosis and in that period I fully educated myself about all relevant aspects of PCa. This meant that when I told them, I could give them an indication of my prognosis, and I could answer pretty much any question they might ask.
In the event, they were quite stunned and didn't ask anything, similarly to what you woild expect when told you have caner, except they had no clue that was coming at all. This actually meant they didn't take in much of what I said, so I made it clear during the following week I was very happy to keep talking about it, so they felt free to ask me more quedtions, which they did.
Some aspects of that will be similar when you tell your children. They won't take it all in first time, and you will need to cover it again in subsequent conversations - make it really clear you expect them to come and ask more about it. Chances are, with 4 children and different ages, they'll handle it differently and want different levels of information at different times.
It's not easy, but as that booklet makes clear, hiding it will cause more problems - hiding something like that from children will fail because they far too good at picking up there's something going on that they're being excluded from, and can leap to all sorts of bad assumptions.
So good luck. The biggest takeaway from telling my parents was that telling them is not the once-off event I had imagined, but an ongoing process.