At this juncture, my advice would be to download the final from here. That's your completed 4K project, and it looks fine. There's no need to start over.
An inspection of the individual transition lengths, shown on each timeline clip, revealed the sole .01 mistake, but that may not be what caused the export issue. "Tahoe" wasn't the cause. Hardware is still suspect. A newer PC is good, but you have not mentioned its specs: CPU/GPU/RAM. Confirm that the video driver and Direct-X are current...
AMD / ATI users: [http://support.amd.com/us/gpudownload/Pages/index.aspx](http://support.amd.com/us/gpudownload/Pages/index.aspx) Intel users: [https://downloadcenter.intel.com/default.aspx](https://downloadcenter.intel.com/default.aspx) nVidia users: [http://www.nvidia.com/Download/index.aspx?lang=en-us](http://www.nvidia.com/Download/index.aspx?lang=en-us) Update DirectX: [http://support.microsoft.com/kb/179113](http://support.microsoft.com/kb/179113)
Whether reducing resolution of the images will help on future projects isn't guaranteed. To accomplish that as a batch would require adding the source files stored on the PC to Prism, selecting the output resolution, and choosing a folder where the results are to be saved. This could take a long time.
The converted files would then be used for the edit. Either 720 or 1080 HD would look fine. The originals won't be affected. They remain in their folder/s.
The regular Prism download is free, but only for a trial period. There is a free release, but some features won't be available, so it may not be suitable.
As for the "Tahoe" fix, simply eliminating the transition at the start of the clip - just making it a cut - might eliminate the glitch.