title: Find a sum x from elements from 4 differens sequences
body: <p>This is My problem:</p>
  
  <p>Given a number x and 4 sequences A, B, C and D, each of n numbers, decide whether there exists some numbers a from A, b from B, c from C and d from D such that x = a+b+c+d. Operations alowed are comparisons, additions and swaps. Design an efficient algorithm to solve this problem with a worst case running time less than n^4.</p>
  
  <p>I have no idea where to start and would appreciate some help!</p>