Quarter-mode cavity resonators in substrate integrated waveguide (SIW) technology represent a good solution for the design of compact filters. However, their advantage in terms of reduced footprint is counterbalanced by possible radiation leakage, which may jeopardize the overall performance. This article presents a systematic investigation of the different contributions of losses in quarter-mode SIW (QMSIW) cavities, considering both the standard topology and modified configurations. General guidelines for the choice of the optimal dielectric permittivity and substrate thickness are derived. Experimental results of individual cavity resonators are presented to validate the simulations. Moreover, the design and the experimental verification of a four-pole filter based on classical and modified QMSIW cavities are reported, to illustrate a possible strategy for performance improvement.