\langle \hat{b}_1\hat{b}_2\hat{b}_3\hat{b}_4 \rangle =\langle \hat{b}_1\hat{b}_2 \rangle \langle\hat{b}_3\hat{b}_4 \rangle +\langle \hat{b}_1\hat{b}_3 \rangle \langle\hat{b}_2\hat{b}_4 \rangle +\langle \hat{b}_1\hat{b}_4 \rangle \langle\hat{b}_2\hat{b}_3 \rangle