The 8 - 4 day is reasonable, just like sending him to daycare for the day, so I don't think that's the issue. Thanks for adding that information to the posting. The dirty toilets and the extra kids hanging around could be the problem. My son is also four years old, and he likes to try before we leave in the morning and at night before bath time. This is by his choice, so that he isn't rushed at other times during the day. At his daycare, he is one of four kids who wait in line to use the bathroom at certain times. My suggestion is that you set up a way for your son to signal an adult to let them know that he needs to "sit". Then, only when the need arises, he could be escorted to the separate bathroom. Other bathroom processes could easily be done with the other kids around (and I don't think that you mentioned any problems in that area, anyway).
Another thought for the future... and I need to do this with my son, too... perhaps you can teach him how to build a nest with toilet paper on the seat. We girls need to know this for public bathroom use because we sit for everything, but the boys may need to know for those times when they can't wait and there are no clean bathroom options available. You got me thinking! Great question! :-)