If W1>2W2, X1=0, X2=y, C(y)=W2*y;
If W1<W2/2, X2=0, X1=y, C(y)=W1*y;
Else if W2/2<=W1<=2W2, solve 2X1+X2=X1+2X2 for X1 and X2, we have X1=X2, thus 2X1+X2=3X1=3X2=y, X1=y/3, X2=y/3, C(y)=(W1+W2)*y/3.
When 2w2=w1<2w3=w4, the firm produces with 2x1+x3 but is indifferent between x1 and x2 and can use any combination of them. In this case, minimum of x1 is 0 and maximum of x1 is y/2. If this is not the case and the firm produces with x3+2x4, its use of x1 is again 0.