  We present new, deep optical observations of the early-type galaxy NGC 4278,
which is located in a small loose group. We find that the galaxy lacks fine
substructure, i.e., it appears relaxed, out to a radius of $\sim$70 kpc. Our
$g$- and $i$-band surface brightness profiles are uniform down to our deepest
levels of $\sim$28 mag arcsec$^{-2}$. This spans an extremely large radial
range of more than 14 half-mass radii. Combined with archival globular cluster
(GC) number density maps and a new analysis of the total mass distribution
obtained from archival Chandra X-ray data, we find that the red GC
subpopulation traces well the stellar mass density profile from 2.4 out to even
14 half-mass radii, while the blue GC subpopulation traces the total mass
density profile of the galaxy over a large radial range. Our results reinforce
the scenario that red GCs form mostly in-situ along with the stellar component
of the galaxy, while the blue GCs are more closely aligned with the total mass
distribution in the halo and were accreted along with halo matter. We conclude
that for galaxies where the X-ray emission from the hot halo is too faint to be
properly observable and as such is not available to measure the dark matter
profile, the blue GC population can be used to trace this dark matter component
out to large radii.
