10x is 10x the output I could have done by myself. 2-4x is between 2 and 4 times the output I could have done myself. I distinguish "value" as that which is strictly a concern for the users/stakeholders, who themselves don't necessarily consider robust testing harnesses or internal-only documentation a value.

I said 4x as a cap for value, I don't know how you interpret that as x^4 ...

4x is 4 * x, x^4 would be your xxx*x ...