Hyperpolarization-activated ion channel that are permeable to Na(+) and K(+) ions with very slow activation and inactivation (PubMed:10228147, PubMed:10430953, PubMed:20829353). Exhibits higher selectivity for K(+) over Na(+) ions (PubMed:10228147). Contributes to the native pacemaker currents in heart (If) that regulate the rhythm of heart beat (Probable) (PubMed:10228147, PubMed:16407510, PubMed:19165230). Contributes to the native pacemaker currents in neurons (Ih) (Probable). May mediate responses to sour stimuli (By similarity). {ECO:0000250|UniProtKB:Q9JKA7, ECO:0000269|PubMed:10228147, ECO:0000269|PubMed:10430953, ECO:0000269|PubMed:16407510, ECO:0000269|PubMed:19165230, ECO:0000269|PubMed:20829353, ECO:0000305|PubMed:10430953}. This is the function of HCN4 (hyperpolarization activated cyclic nucleotide gated potassium channel 4, ENSG00000138622).