I had an issue recently - don't know what caused it because it also effected my Music app, Calendar app and the Photos app. (I think it was something with the Apple servers and after dealing with several Senior Techs it just seemed to "clear up". What you can try is create a Smart Album and set the parameters to "Photo is Unable to Upload to iCloud Photos". This may show you the photos that for some reason - assuming the you started with the Library on your computer - can't upload. Could be the file format or some other reason that it couldn't be uploaded and then won't show on your other devices libraries.
That would be the easiest way - assuming the same four photos are missing from the other devices - but worth a try before trying to re-sync or re-download the photos again.
There is also a program that people use to merge and check photo libraries for duplicates and other issues. I had it years ago - specifically for finding duplicates - but I don't see installed any longer. You can check out what it is capable of or do searches of the various photo forums for people that use it.
www.fatcatsoftware.com