var str = "87b86156d0970a4200005e02612f56614f7a2f54f7ebf45670ed62cbaa78e6f228297b0e7338215fb4";

​

var grabLength = 4,

    // 4-char strings

    grp1 = [],

    // 2-char strings

    grp2 = [],

    chunk;

​

while(chunk = str.slice(0, grabLength)) {

  str = str.slice(grabLength);

  // skip all zeros

  if (/^0+$/.test(chunk)) continue;

  if (grabLength === 4) {

    if (grp1.push(chunk) === 5) {

      grabLength = 2;

    }

    continue;

  }

  // skip 2-char sequences that match the start of a 4-char sequence

  var hasMatch = false;

  for (var i = 0; i < grp1.length; i++) {

    if (chunk === grp1[i].slice(0, 2)) {

      hasMatch = true;

      continue;

    }

  }

  if (hasMatch) continue;

  grp2.push(chunk);

  if (grp2.length === 2) break;

}

​

console.log(grp1);

console.log(grp2);

​

​