  We develop a technique to measure radial velocities of stars from spectra
that present four sets of lines. The algorithm is an extension of the
two-dimensional cross-correlation method TODCOR to four dimensions. It computes
the correlation of the observed spectrum against a combination of four
templates with all possible shifts, and allows also for the derivation of the
light ratios of the components. After testing the algorithm and demonstrating
its ability to measure Doppler shifts accurately even under conditions of heavy
line blending, we apply it to the case of the quadruple-lined system HD 110555.
The primary and secondary components of this previously known visual binary
(separation about 0.4 arcsec) are each shown to be double-lined spectroscopic
binaries with periods of 57 days and 76 days, respectively, making the system a
hierarchical quadruple. The secondary in the 76-day subsystem contributes only
2.5% to the total light, illustrating the ability of the method to measure
velocities of very faint components.
