The present invention relates to novel compounds and probes which have a common chemical structure necessary to obtain potent inhibitory activity against KLK4 and/or may be used for the detection of KLK4 peptides and their activity. It further relates to the use of these compounds and methods for inhibiting and/or detecting KLK4 activity in vitro and in vivo by making use of said probes or inhibitors. The compounds of the invention differ from prior art compounds at least in the presence of phenyl guanidine (instead of e.g. benzyl guanidine) and/or the presence of a heteroatom in the tail group, their combined presence unexpectedly leading to potent and selective KLK4 inhibitory activity.