  We investigate the current-current correlations in a four-terminal Al-AlOx-Al
tunnel junction where shot noise dominates. We demonstrate that
cross-correlations in the presence of two biasing sources of the Hanbury-Brown
and Twiss type are much stronger (approximately twice) than an incoherent sum
of correlations generated by single sources. The difference is due to voltage
fluctuations of the central island that give rise to current-current
correlations in the four contacts of the junction. Our measurements are in
close agreement with results obtained using a simple theoretical model based on
the theory of shot noise in multi-terminal conductors, generalized here to
arbitrary contacts.
