Huge UHD files can be problematic. In most cases, they're not needed. Suggest shooting in or converting to 1920 x 1080 HD which usually gives excellent results.
Prism is an NCH conversion tool, but this can also be handled - file-by-file - in Videopad itself via export settings. Your mp4 files were converted to HD and substituted in the project, which then exported without crashing.
As a matter of practice, consider converting files to low res "proxies" and placing them in a folder separate from the originals. Video editing is resource intensive. Time spent up front will pay off handsomely during the edit.
When satisfied with the project, right-click on each source file in the bin and use FILE|CHANGE FILE to substitute the HD originals. Alternatively, if the proxies and originals have exactly the same names, they can be swapped out with a file manager. VP will recognize the files that it finds in its project folder. Take care that the two sets of files are independently backed up..