This is the big one! Travelling throughout the region, you'll touch on the highlights of all four countries, including Bangkok and Sukhothai in Thailand, Luang Prabang and Four Thousand Islands in Laos, Phnom Penh and Angkor Wat in Cambodia, and of course, Saigon, Hoi An and Hanoi in Vietnam.
For those set on seeing all four countries, this is a good route to take as it has a minimal amount of backtracking. If you cross from Laos to Vietnam by one of the more central crossings on the panhandle (such as Lao Bao or Lak Sao), you will need to do some substantial backtracking legs in Vietnam if you want to see the entire country.
There's a load of travel involved in a circuit like this -- particularly if you decide to do the whole trip overland. If you're a bit short of time or just can't face getting on another bus, look into flying some of the legs.
Suggest trip length
If you're willing to fly a few routes and skip out some of the lesser-known spots, you could cover all four countries in as little as two weeks, but we'd say that was a really bad idea -- unless your idea of a holiday is to be on the move just about every day. Realistically a comfortable minimum to do the following route, with stops at all the designated towns, would be at least five to six weeks. With longer stays in a few spots and perhaps with a few extra add-ons, eight to ten weeks would be a comfortable pace of travel. If you decide to add in the South of Thailand (not covered on this trip plan) add in another two weeks. Thailand and Vietnam in particular are skipped through in this trip -- one could easily spend far longer in each country.