Southeast Asia forum

Such an open-ended question is not that easy since you really have to pick your regions.

I will give you 1 suggestion and hopefully for you others will jump in with theirs. Mine is by no means the perfect one but since I am a food and market enthusiast as well this is what I would do.

I have no good recent knowledge about Malaysia or southern Thailand but I assume you could do some of that region in your first two weeks.

From there starting in Bangkok you could do in your last two weeks:
- few days North eastern Thailand around Khao Yai national Park. No need to go into the park (timewise I mean) but the scenery around there is beautiful. Also the towns are pleasant and have nice little local markets. Food is also good there (as everywhere in Thailand)

- then Siem Reap /Angkor in Cambodia for a few days for a dose of culture and admire the temples. Most restaurants in Siem Reap cater for tourists but you might have a craving for some western food by then and there's also plenty of Khmer food to be tried. Although tasty, the Cambodian food doesn't match Thai or Vietnamese food in uniqueness.

- last week start to travel down to Kep (via Phnom Penh) in 2-3 days and spend 1 relaxing day by the coast and eat some fresh crab & kampot pepper at one of the restaurants there.

- last weekdays cross the border via Ha Tien and travel through the Mekong Delta towards HCMC. Mekong Delta has some great real (floating)markets, fruit & rice farms and great river scenery.

- last day in HCMC you can try some truly great Vietnamese food and then fly out to KL

You can't see all and this will give you a taste (literally) of the region. Leave northern Thailand, Laos and rest of Vietnam for another visit. But others will disagree and might have a bettter plan.
With an itinerary like this your travel times/days are manageable and no need for a lot of flights