Pore-forming subunit of Nav1.4, a voltage-gated sodium (Nav) channel that directly mediates the depolarizing phase of action potentials in excitable membranes. Navs, also called VGSCs (voltage- gated sodium channels) or VDSCs (voltage-dependent sodium channels), operate by switching between closed and open conformations depending on the voltage difference across the membrane. In the open conformation they allow Na(+) ions to selectively pass through the pore, along their electrochemical gradient. The influx of Na+ ions provokes membrane depolarization, initiating the propagation of electrical signals throughout cells and tissues (PubMed:12766226, PubMed:15318338, PubMed:16890191, PubMed:17898326, PubMed:18690054, PubMed:19347921, PubMed:25707578, PubMed:26659129, PubMed:26700687, PubMed:29992740, PubMed:30190309). Highly expressed in skeletal muscles, Nav1.4 generates the action potential crucial for muscle contraction (PubMed:16890191, PubMed:19347921, PubMed:25707578, PubMed:26659129, PubMed:26700687). {ECO:0000269|PubMed:12766226, ECO:0000269|PubMed:15318338, ECO:0000269|PubMed:16890191, ECO:0000269|PubMed:17898326, ECO:0000269|PubMed:18690054, ECO:0000269|PubMed:19347921, ECO:0000269|PubMed:25707578, ECO:0000269|PubMed:26659129, ECO:0000269|PubMed:26700687, ECO:0000269|PubMed:29992740, ECO:0000269|PubMed:30190309}. This is the function of Ensembl gene identifier ENSG00000007314 (SCN4A, sodium voltage-gated channel alpha subunit 4).