The outputs are created according to the formula $$y = (x1^2 + (x2 x3 - (1/(x2 x4)))^2)^{0.5} + e$$ where e is N(0,sd).
mlbench.friedman2(n, sd=125)
Friedman, Jerome H. (1991) Multivariate adaptive regression splines. The Annals of Statistics 19 (1), pages 1-67.