i think its safe to assume you are going to be painting everything you assemble and vice-versa, so:

X1=X3, X2=X4.

also, you cant have negative numbers:

There are 2 more constraints i can think of:

what can you say about the relationship between X1 and X2? (hint: inequality)

what can you say about the relationship between X3 and X4? (hint: inequality)