foo.js:

function medeltal(tal1, tal2, tal3, tal4)
{
	var svar = (tal1 + tal2 + tal3 + tal4) / 4;

	return svar;
}


bar.php:

                tal1 = parseInt(prompt("Ange tal: "));
		tal2 = parseInt(prompt("Ange tal: "));
		tal3 = parseInt(prompt("Ange tal: "));
		tal4 = parseInt(prompt("Ange tal: "));	

		medeltal(tal1, tal2, tal3, tal4);
		
		summa = medeltal(svar);

		document.write(summa);