Hi! Not sure if you have your answer yet, but I was having the same problem tonight, and was able to correct most of it, granted you are working with a lot more photos than I had, so the solution might be extremely tedious for you. I had copied 6900 photos from an old iPhoto library into a folder on my new computer's desktop, and wanted to import them into my new Photos library. I got the same error message (4000 would not import), tried to mess with the permissions, etc., and nothing worked. I started moving around my files a bit, and for some reason, I got most of them to import (I honestly have no idea why) using the following steps:
• I created a separate folder on my desktop entitled "unimported."
• I clicked "import" in Photos, and imported one group of pictures at a time, alphabetically (first I did all the A's, then the B's, etc...like I said, tedious....)
• When I got the error message, I grabbed all of the files mentioned in the error and put them into the new "unimported" folder. Since they weren't listed alphabetically, and to make it easier to see which files to move, I copied the list of files on the error message and pasted it into a spreadsheet where I could alphabetize the names. Again, tedious...
• After going through the entire alphabet of photos and separating the unimported ones, I again hit "import" in Photos and selected all of the files in my "unimported" folder. Miraculously, most of them imported.
• Any files that still came up in the error message, I placed into a new "Still Unimported" folder.
• Once again, I hit "import" in Photos, and selected all of the "Still Unimported" files. I got all of them to import except 14 out of the 6900, and these 14 files also won't open in Preview, so they are most likely corrupted. I also wasn't able to import about 10 video files, but they will open in quicktime.
Not sure if this will help, but it seemed that for some weird reason, moving the files to different folders helped me! There has got to be an easier way, but not sure what it is. Good luck!