Visiting 4-6 countries in East Asia/Southeast Asia possible in 4 weeks?

Hey guys, I'm planning to take a big trip to East Asia and Southeast Asia sometime this year! I wanted some help and suggestions on places to go. I have about 28 days (4 weeks) I can use to do the trip.

Here is my rough itinerary so far:

• Japan (10-12 days). I've already been to Japan but I loved my first visit so I want to visit again.
• Vietnam (7-10 days). This will be my first time in Vietnam as an adult. But like Japan, Vietnam is a country I want to definitiely visit when I'm in Asia.

Other places I want to visit, if I have time:

• Hong Kong (loved it the first time and it's been over 10 years since i last went, so I want to go again)
• Shanghai and Beijing in China: I've never been, but was always interested in them
• Philippines: Never been, but I heard it's really cheap and lots of beauty.

These are maybes for me:

• Thailand: I heard Thailand is nice and the food is great, but I guess I'm not that interested
• South Korea: I've already been to South Korea, it was alright but not a big deal for me
• Taiwan: Taiwan is beautiful but I've already visited so it's not top priority for me

Right now, here's my plan:

• 10-12 days in Japan
• 7-10 days in Vietnam
• 2-3 days in Hong Kong
• 2-3 days somewhere else in Asia

What do you guys think? Am I stretching myself too far? Or do you have other places I should visit instead? Would love to hear your suggestions.