The function of KCNIP4 (potassium voltage-gated channel interacting protein 4, Ensembl gene identifier ENSG00000185774) is as follows. Regulatory subunit of Kv4/D (Shal)-type voltage-gated rapidly inactivating A-type potassium channels. Modulates KCND2 channel density, inactivation kinetics and rate of recovery from inactivation in a calcium-dependent and isoform-specific manner (PubMed:11847232, PubMed:18957440, PubMed:23576435). Modulates KCND3/Kv4.3 currents (PubMed:23576435). Isoform 4 does not increase KCND2 expression at the cell membrane (PubMed:18957440). Isoform 4 retains KCND3 in the endoplasmic reticulum and negatively regulates its expression at the cell membrane. {ECO:0000250|UniProtKB:Q6PHZ8, ECO:0000269|PubMed:11847232, ECO:0000269|PubMed:18957440, ECO:0000269|PubMed:23576435}.