Tortoise - firstly, well done for being brave. That was a hard, hard phonecall.
Secondly, ignore all the bitching and fighting on here. These people have their own experiences, and their own 'baggage' to deal with. You have your situation to deal with.
Thirdly, if you stop now, and you find that in the future your children, all of them, are emotionally scarred, you will live with that forever.
Think of it like a wound with grit and dirt in it. It will heal eventually, or seem to. But that grit will eventually work its way to the top and burst out, and the result is not pretty.
A wound like that needs to be washed out, cleaned, and carefully stitched so that the scar is neat and strong.
Your children, all of them, DD1, DD2, DS1 and now DS2, need you to show them how you deal with difficult situations. You don't run away, you don't hide it, you confront it.
Your DS1 has been a victim once. Don't let him be a victim of himself now. Let him get the help he needs.
Your DD1 & DD2 need to know that when they turn to their Mum for help, they get it, no matter how hard it is.
Your DS2 is watching and listening, you can be sure. He needs to know too, that any sexually inappropriate behaviour is dealt with immediately, dealt with properly and is NOT acceptable. The alternative is that you teach him that girls aren't that important.
You CAN DO THIS. You HAVE to do this.
Think on. Do you want to be in the same situation next year? Do you want to worry for the rest of your life about your DS's behaviour, and your DD's future?
Now think about your DS having had help to change his behaviour, change his thought processes, become a wholesome, decent young man who would be an absolute treasure to a young lady in the future. And a DD1 and DD2 who went into adulthood free from the shadow of this. Who could trust and love a man, without fearing abuse.
Take care, and remember that you can do this.