You should use the Blender video editing mode for exporting videos.
Then press Shift-A to get drop down add-file menu, or click on the "Add" button.
(Sorry about the sloppy arrow. I didn't have enough time to draw a good one)
select the "Image" button and go to the file in the file browser that you want to add frames from, and press A to select all. If you ...
It simply means you didn't clicked on solve button after tracking your video
steps to have a clean track
Track points on your video
Highlight all track markers or your points by pressing A and pres Ctrl + L
Now click on Solve camera. Aim to get an error below 0.3 pixels, else refine you tracking.
Now click on the center most point and click Set origin
Either there is something different in 1.mp4 or there something wrong in the Blender import of them.
Try to compare the metadata of the clips in MediaInfo if they are similar, if they're not, then that might be the reason.
If they are similar, try to make four copies of one of the good clips, and import them in Blender, if the problem is visible, then do ...
I you loose sync between video and audio, generally the reasons can be:
1) you didn't match the frame rate (in your case 29,97)
2) you didn't match the audio sample rate (in your case 48 KHz), in the preferences
The others parameters are a matter of quality/file weight, not about Sync.