The function of Ensembl gene identifier ENSG00000135643 (KCNMB4, potassium calcium-activated channel subfamily M regulatory beta subunit 4) is as follows. Regulatory subunit of the calcium activated potassium KCNMA1 (maxiK) channel. Modulates the calcium sensitivity and gating kinetics of KCNMA1, thereby contributing to KCNMA1 channel diversity. Decreases the gating kinetics and calcium sensitivity of the KCNMA1 channel, but with fast deactivation kinetics. May decrease KCNMA1 channel openings at low calcium concentrations but increases channel openings at high calcium concentrations. Makes KCNMA1 channel resistant to 100 nM charybdotoxin (CTX) toxin concentrations. {ECO:0000269|PubMed:10692449, ECO:0000269|PubMed:10792058, ECO:0000269|PubMed:10828459}.