Internal function used compute a sum in FPCA-based covariance updates
f_sum4(mu.q.c, sig.q.c, mu.q.bpsi, sig.q.bpsi, theta, obspts.mat)
current value of mu.q.c
current value of sig.q.c
current value of mu.q.bpsi
current value of sig.q.bpsi
current value of theta
matrix indicating where curves are observed