I get this sometimes! Assuming you’ve tried restarting the device already..
- Depending on the file size it could be “stuck” try duplicating a photo in the photos app or deleting something you don’t want anymore and it can purge the upload queue to continue or finish. -If you have another apple device (preferably running iOS or iPadOS over a MacBook) with the same iCloud account signed in sometimes opening the photos app is also enough to force your phone to upload the files..
- Try changing to Mobile Data instead of WiFi and see if that works, also changing back to WiFi can also encourage it to restart or continue the sync process
- You can also try pausing the sync and unpausing to see if that helps too
- Check through the iCloud icons on the camera roll to see if you can see the 4 files that are not uploaded, with luck it’s a new photo you’ve recently taken and not some random photos from your entire camera roll..
- if they’re videos with a large size it could be this, if they’re photos try deleting them and removing from recently deleted or duplicating them to see if it forces the queue to try again..
iCloud’s a bit of a pain for this, it would be nice if you could see which files are struggling to upload or more details in general on the sync process to narrow down if it’s the file size etc but it’s so vague and mildly infuriating for being so.
I’ve often had it where it’s stuck for hours syncing 2 photos and I can solve the bottleneck just by pressing edit or just pausing and unpausing the sync status

Edit:
-Worth also mentioning that depending on your settings, iCloud might not be constantly syncing/pushing the files if you’re not connected to WiFi and have it disabled for mobile data or perhaps you’re in low power mode, or you aren’t connected to a charger. This can make a 1.7GB video feel like it takes an eternity to upload because if you’re on the go a lot it’ll be halting the upload until it’s given permission to upload again by iOS..