x1^2+x2^2+x3^2+x4^2+x1x2+x1x3+x1x4+x2x4+x3x4
= (x1/2+x4/2+x3)^2+(x1/2+x4/2+x3)^2 + x1^2/2+x4^2/2
≥ 0