Essentially, you cannot. Or constraints are not an option for any optimizer, including GA, thus x1>x2 OR x1<x2. And you cannot have strict inequalities anyway.
Are these integers?
Could you require that the x_i are sorted, thus something like:
x1 + tol <= x2
x2 + tol <= x3
x3 + tol <= x4?
Yes, somebody will probably tell you to use a constraint like
abs(x1 - x2) >= tol
etc.
It might even work, SOME of the time, IF you used good starting values. But it will certainly create a problem with multiple local minima, ABSOLUTELY necessitating good starting values.
