Regulatory subunit of the slx1-slx4 structure-specific endonuclease that resolves DNA secondary structures generated during DNA repair and recombination. Has endonuclease activity towards branched DNA substrates, introducing single-strand cuts in duplex DNA close to junctions with ss-DNA. Has a preference for stem-loop (SL) and splayed arm Y structures. Introduces a single-strand cut in duplex DNA on the 3' side of a double-strand/single-strand junction with respect to the single-strand moving 3' to 5' away from the junction. Plays a critical role in maintaining the integrity of the ribosomal DNA (rDNA) loci, where it has a role in re-starting stalled replication forks. The complex initiates homologous recombination (HR) events, used to maintain rDNA copy number, in the rDNA repeats that are processed by a mechanism that requires rad22, but not rhp51. Has Holliday junction resolvase activity in vitro. Slx4 is required for efficient processing of DNA substrates. Forms a heterodimer with slx1. Associates with chromatin at rDNA repeat protrusions. Phosphorylated in response to DNA damage. Simultaneous elimination of slx4 and rqh1 is lethal. Belongs to the SLX4 family.