score:7
With only four days, you are better off exploring the country where you land. There are buses between Vietnam and Cambodia (no trains), but you will waste two of your days dealing with bus transport between the countries. You can fly to Cambodia (Siem Reap or Phnom Penh) from Saigon or Ha Noi, saving lots of time, but costing a lot more money.
Bottom line to visit Angkor Wat with only four days you need to bite the bullet and fly to Siem Reap.