This study presents a compact, wideband, 4 × 4 Butler matrix using a two-metal-layer thin-film process for millimeter (mm)-wave 5G radio applications. Size reduction techniques are thoroughly discussed for size and performance optimization. A size-reduced quadrature coupler is designed with coplanar waveguide based length-reduced branch lines using capacitive loading effect from the ground bridges. A compact crossover is designed for a return loss of 20 dB or more with negligible crosstalk over a wide frequency range. The circuit components are arranged in an optimized space, allowing negligible coupling between nearby elements. The Butler matrix is fabricated on a quartz substrate at the center frequency of 28 GHz. The fractional bandwidth is 15.4% at a 15-dB return loss and insertion loss is measured in the range of 7.8–8.7 dB at the center frequency. The size of the core circuit is 3.375 × 2.100 $${\rm{m}}{{\rm{m}}}^{2}$$, which corresponds to 0.469 × 0.292 $${\lambda }_{g}^{2}$$.