  We have used a combination of broad-band near-infrared and optical
Johnson-Cousins photometry to study the dust properties in the line of sight to
the Galactic globular cluster M4. We have investigated the reddening effects in
terms of absolute strength and variation across the cluster field, as well as
the form of the reddening law defined by the type of dust. Here, we determine
the ratio of absolute to selective extinction (R_V) in the line of sight
towards M4, which is known to be a useful indicator for the type of dust and
therefore characterizes the applicable reddening law. Our method is independent
of age assumptions and appears to be significantly more precise and accurate
than previous approaches. We obtain A_V/E(B-V)=3.76\pm0.07 (random error) for
the dust in the line of sight to M4 for our set of filters. With this value,
the distance to M4 is found to be 1.80\pm0.05 kpc (random error). A reddening
map for M4 has been created, which reveals a spatial differential reddening of
delta E(B-V)>0.2 mag across a field within 10' around the cluster centre and a
total mean reddening of E(B-V)=0.37\pm0.01. In order to provide accurate zero
points for the extinction coefficients of our photometric filters, we
investigated the impact of stellar parameters such as temperature, surface
gravity and metallicity on the extinction properties in different bandpasses.
Using both synthetic ATLAS9 spectra and observed spectral energy distributions,
we found similar sized effects for the range of temperature, surface gravity,
and metallicity typical of globular cluster stars: each causes a change of
about 3% in the necessary correction factor for each filter combination. From
our calculations, we provide extinction zero points for Johnson-Cousins and
2MASS filters, spanning a wide range of stellar parameters and dust types,
suited for accurate, object-specific extinction corrections.
