Bellman-Ford pass 1:  h(1)= 0  h(2)= 0  h(3)=-2  h(4)= 0
Bellman-Ford pass 2:  h(1)= 0  h(2)= 0  h(3)=-2  h(4)= 0
Bellman-Ford pass 3:  h(1)= 0  h(2)= 0  h(3)=-2  h(4)= 0
Bellman-Ford pass 4:  h(1)= 0  h(2)= 0  h(3)=-2  h(4)= 0

reweighted edges:
  w'(1,2) =  4 + ( 0) - ( 0) = 4
  w'(1,4) =  5 + ( 0) - ( 0) = 5
  w'(2,3) = -2 + ( 0) - (-2) = 0
  w'(3,1) =  3 + (-2) - ( 0) = 1
  w'(3,4) =  2 + (-2) - ( 0) = 0
  w'(4,2) =  1 + ( 0) - ( 0) = 1
  w'(4,3) =  6 + ( 0) - (-2) = 8

  source   dhat(u,1..4)          D(u,1..4)
       1      0    4    4    4         0    4    2    4
       2      1    0    0    0         1    0   -2    0
       3      1    1    0    0         3    3    0    2
       4      2    1    1    0         2    1   -1    0
