Hi Louise, such a hard situation! However, I think the best thing would be to be as honest as possible, as soon as possible. The longer you leave it, the harder it will become. If your situation allows, could you get all your sons together (maybe over dinner?) and tell them all one night? (It will save having to say it 4 times). Do they know you've seen a doctor at all? If not, start with that, then just tell them what unfolded.
Allow time for the inevitable questions and expressions of disbelief (you may even have to repeat it a few times)but please don't put it off for too long. It will be difficult, but it has to be done. You could even post on here straight after, if you need a debrief. We'll all be here for you. love Emily