I had a 4yo who would do that as well. Turns out he would get distracted looking at other things or be in a rush to get back to playing. Every time he went to the bathroom I would have to remind him to focus on what he was doing and then I would go in and check afterwards to make sure that he got it all in the toilet. The other thing that made him pay more attention was the fact that I made him clean up his mess and then afterwards I would make him watch me thoroughly clean the toilet area again explaining that he was a big boy and that it wasn't fair that I had to clean up the bathroom every time he took a pee. It took about 3 times and it hasn't happened in months.