How to define if/else non linear constraint in an optimization problem

if x(1)+x(2)+x(3)= 0 then x(4)= 0

elseif 0<x(1)+x(2)+x(3)<20 then x(4)=10

elseif 20<x(1)+x(2)+x(3)<60 then x(4)=30

Answer 1

[Bruno Luong]

Simply solve three different optimization pbs with the three repectively constraints independently and ignore the two other, then select the best of the three solutions.