Wilder signed a four year deal in 2020 it had roughly 3 years to run so the £4m would have been the balance of the contract which is what Sheffield United would have had to pay him. They wanted him to stay but they wanted him to work with a DoF and looking at the absolute black hole their recruitment has been that is not wholly unreasonable. If he wanted resign fine not a problem but not a penny is due in compensation.
If the interview is half true I wouldn't touch Wilder with a 10 foot pole.