algo.js

const assert = require('assert');

function solution(array) {
  const k = array.length / 4;
  let buffer = [];
  return flatten(array.reduce((a, b, i) => {
    buffer.push(b);
    if (b !== array[i + 1]) {
      a.push(buffer);
      buffer = [];
    }

    return a;
  }, []).filter(x => x.length === k));
}

function identical(a) {
  return a.every(x => x === a[0]);
}

function flatten(a) {
  return a.reduce((a, b) => a.concat(b), []);
}

assert.deepEqual(solution([1, 1, 2, 2, 2, 3, 3, 4, 5, 5, 5, 5, 6, 6, 6, 6]), [5, 5, 5, 5, 6, 6, 6, 6]);
assert.deepEqual(solution([1, 1, 2, 3, 4, 5, 6, 7]), [1, 1]);