In a situation where the software is only used internally, it sounds like it would be very easy to game what "4% of software's revenue" means in both directions.
E.g. Unity argues that the entire production plant is the result of the software, therefore charges 4% on the revenue of the factory, meanwhile the customer argues that the software produces no revenue, because it is not being sold and therefore the bill is $0.
E.g. Unity argues that the entire production plant is the result of the software, therefore charges 4% on the revenue of the factory, meanwhile the customer argues that the software produces no revenue, because it is not being sold and therefore the bill is $0.
A fixed monthly fee solves this problem.