  We present a four-point probe resistance measurement technique which uses
four equivalent current measuring units, resulting in minimal hardware
requirements and corresponding sources of noise. Local sample potentials are
measured by a software feedback loop which adjusts the corresponding tip
voltage such that no current flows to the sample. The resulting tip voltage is
then equivalent to the sample potential at the tip position. We implement this
measurement method into a multi-tip scanning tunneling microscope setup such
that potentials can also be measured in tunneling contact, allowing in
principle truly non-invasive four-probe measurements. The resulting measurement
capabilities are demonstrated for BiSbTe$_3$ and Si$(111)-(7\times7)$ samples.
