  In this paper, we present results from the spectropolarimetric follow-up of
photometrically selected candidate magnetic B stars from the MOBSTER project.
Out of four observed targets, one (HD 38170) is found to host a detectable
surface magnetic field, with a maximum longitudinal field measurement of
105$\pm$14 G. This star is chemically peculiar and classified as an $\alpha^2$
CVn variable. Its detection validates the use of TESS to perform a photometric
selection of magnetic candidates. Furthermore, upper limits on the strength of
a putative dipolar magnetic field are derived for the remaining three stars,
and we report the discovery of a previously unknown spectroscopic binary
system, HD 25709. Finally, we use our non-detections as case studies to further
inform the criteria to be used for the selection of a larger sample of stars to
be followed up using high-resolution spectropolarimetry.
