Have you thought of the possibility that you might be having a few mild spasms in your esophagus? I know that they don't fit the pattern of all-out spasms, but that's the thought that came to my mind.
Everyone's recovery is SO different. I remember speaking to my surgeon when he said that just when he thought he has heard every possible post-op symptom, someone comes in with a new one.
You're only 4 weeks post op. According to my surgeon, "pretty much anything is possible during the early stages of recovery."
Have you called your surgeon's office? If you're really concerned, go for it. Just know that you're healing inside, and although your outsides healed quickly, your insides take a LOT longer. I was told 6 weeks for most of the healing and a year for the rest. I have found that I have continued to improve over the second year as well, believe it or not. It takes time for your body to adjust to a brand-new, revised upper GI tract.
As I said, if you are worried, you should feel free to talk to someone in your surgeon's office. (Although from experience, it is often not a very satisfying conversation! )
Take care and hang in there.
It's a challenging recovery, but very manageable.
It will get better!