Essentially the same result here, when loaded onto an SSD; the project appears to be unwieldy. There are numerous unused files in the media bin (no green check mark) and major gaps in the video track. Though the Mp4 files have an audio track, you don't seem to need it.
The bit rate of the Mp4 files is in the mid six digits (3840 x 2160 @ 59.94): VP may have a problem with that. You should get better results with 1920 x 1080 @29.97 or 30 fps for your source files.
To salvage the existing, loaded project, try this...
Click the + sign to the right of the sequence 1 tab on the timeline to open a new blank sequence.
Load the files that will be used for the first thirty seconds of so of video. Then, for each clip, right-click the audio track and UNLINK from video. Then right-click|DELETE the audio.
Let the green line complete. Now export sequence 2 at 1920 x 1080 @ 29.97 fps. Give it a unique number. such as "sequence 1, 2, etc." Loss of quality should be negligible when played on a PC. This becomes a simple video only clip - likely in your DOWNLOADS folder - combining the files that were used to create it.
Once exported, delete all files in sequence 2. Load the next thirty seconds or so of clips into that sequence. Repeat the load|export process until you have processed all the needed clips.
Finally, clear sequence 2. Add your new Mp4 files to the media bin. Place them on the time line in order. Add your music track. There now should be a single video and a single audio track. When sequence 2 is exported, the entire project will be the result.