Welcome to a household of boys! I dont ever seeing this "issue" going away. When they were young, my boys would play "sword" - and always had "over spray". I did teach all of them on how to wipe all around the toilet after they do their business and also taught them to put the toilet seat down. I don't know how many times my girls when they were little fell into the toilet during the night, so this was an ongoing training issue until the boys figured it out that I wasn't going to let up. This was not an option in my household. With your Son 4 years old, it maybe a good time to begin teaching him to clean up after himself. By the time your daughter reaches age to be independent in the bathroom, your son will be well taught that this is all part of the process of doing their business in the bathroom and your daughter won't fall into the toilet.
With all my sons being adults, when they come to visit, I cannot tell when I walk in the bathroom that a male was in the house. As for my husband, he has been "trained" for many years. lol
I think of it this way, why do the public bathrooms have urinals for men? Because it is obvious that males need assistance so there is no overspray.
"Just thinking out loud here - hope this assists"
Mom of 6
4 adult sons, 2 daughters