Associates with and regulates the activity of the sodium/potassium-transporting ATPase (NKA) which catalyzes the hydrolysis of ATP coupled with the exchange of Na(+) and K(+) ions across the plasma membrane (By similarity). Increases the apparent affinity of the transporter for Na(+) and increases NKA activity (By similarity). . This is the function of FXYD4 (FXYD domain containing ion transport regulator 4, ENSG00000150201).