Sounds like you have only the best in mind for that little boy, great job! It's sad, but you can really only do so much; you can't control what she chooses to do/say at her house. At 4 they can understand quite a bit, but usually they don't need that many details. The most simple explanations are best. I would suggest asking him for questions. He may only have a few (like what those words mean).. And most importantly, assure him that although parents may choose to not be married anymore (if they are not in love with each other anymore) that a parent doesn't ever stop loving their child... no matter what.
Good luck and welcome to the journey of parenting! It will be hard but worth it!