unable to understand lowest sum logic

I am a newbie to codechef.I am unable to understand logic for lowest sum.I also read the editorial still couldn’t get it.Can anybody explain it to me?
http://www.codechef.com/problems/LOWSUM

u get 2 arrays of ints. find the 4th lowest sum of 2 ints ->1 from each array. Its that simple

Your code would fail for boundary conditions and would so give an runtime error.

what are such border conditions according to you?