I was looking at some photos and Halong Bay and Hanoi seems nice. anyway since Azure has some connections it would definitely be easier. I will discuss with him, maybe meeting on Mon or Tue to discuss and come up with something more concrete. .
Do go to the NORTH if u can -it's more interesting than the South ( HCM city area) but 4 days wouldn't do it justice. Check out the colourful weekend markets near SAPA like Bac Ha,Coc Ly,etc. Sapa is good as a base but it's market being overun n is least interesting.
If u have only 4 days better just concentrate on Halong bay n Hanoi.