Can a principal that was paid on the wrong salary base for the past 4 years be paid the difference by her district? She was not aware of this problem until this past month. This error was made by the business manager.

Can she? Sure. Is the district obligated to correct the mistake going back 4 years? No unless there is a contract that states she must be paid a certain amount and she was not. It doesn't matter who made the mistake. The employee always has an obligation to bring these things to their employer's attention as mistakes do happen. If she let it go for 4 years, that is akin to agreeing with the salary paid.

Again, school employees often operate under contracts and there *may* be a provision in the contract that governs this issue. In that case, the contract takes precedence.