f=(x1-x2)^2 - (x3+2x4)^2 +2x4^2
= y1^2-y2^2+2y3^2
y1=x1-x2
y2=x3+2x4
y3=x4
y4=x2
Y=CX,C=
1 -1 0 0
0 0 1 2
0 0 0 1
0 1 0 0