const nestedArray = [ [ 'm', 1 ], [ 'i', 4 ], [ 's', 4 ], [ 'p', 2 ] ];

function descNumbersAscChar(arrA, arrB) {
  if (arrA[1] > arrB[1]) { return -1 }
  else if (arrA[1] === arrB[1]) {
    if (arrA[0] < arrB[0]) { return -1 }
    else if (arrA[0] > arrB[0]) { return 1}
  }
  else if (arrA[1] < arrB[1]) { return 1 }
  else {
    console.log('Uncaught situation!')
    return 1
  }
};

console.log(nestedArray.sort(descNumbersAscChar)); // [ [ 'i', 4 ], [ 's', 4 ], [ 'p', 2 ], [ 'm', 1 ] ]