While rare-earth magnets exhibit unchallenged hard-magnetic properties, looking for alternatives based on inexpensive elements of non-critical supply remains of utmost interest. Here, we demonstrate that (Fe,Co)₂(P,Si) single crystals combine a large magnetocrystalline anisotropy (K₁ ≈ 0.9 MJ m⁻³ at 300 K), high Curie temperatures (T_C up to 560 K) and an appreciable saturation specific magnetization (101 A m² kg⁻¹) leading to a theoretical |BH|_max ≈ 165 kJ m^-3, making them promising candidate materials as rare-earth-free permanent magnets. Our comparison between (Fe,Co)₂P and (Fe,Co)₂(P,Si) single crystals highlights that Si substitution reduces the low-temperature magnetocrystalline anisotropy, but strongly enhances T_C, making the latter quaternary alloys most favorable for room temperature applications. Submicron-sized particles of Fe_1.75Co_0.20P_0.75Si_0.25 were prepared by a top-down ball-milling approach. While the energy products of bonded particles are to this point modest, they demonstrate that permanent magnetic properties can be achieved in (Fe,Co)₂(P,Si) quaternary alloys. This work correlates the development of permanent magnetic properties to a control of the microstructure. It paves the way toward the realization of permanent magnetic properties in (Fe,Co)₂(P,Si) alloys made of economically competitive Fe, P and Si elements, making these materials desirable for applications.