По-поему так проще всего
var a = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49];
var b = [];
for (var i = 0; i < a.length; i += 4) {
b.push(a.slice(i, i + 4));
}