I edit longer videos in chunks and stitch them together on occasion too. I use ffmpeg command line to do the exact same thing as sauron’s excellent suggestion. The advantage I find with the command line is that I can script the stitch process and re-run it many times as I make tweaks to each chunk. avidemux can probably do this too, but I haven’t researched it.
The ffmpeg method involves putting your four videos in a folder. In that same folder, create a text file called filelist.txt and put these lines in it:
file 'Part1.mp4'
file 'Part2.mp4'
file 'Part3.mp4'
file 'Part4.mp4'
The files are encoded in final delivery format (YouTube preset in your case). Then ffmpeg is called to stitch all the entries in filelist.txt into a single file without re-encoding:
ffmpeg -f concat -safe 0 -i filelist.txt -c copy "FinalOutput.mp4"