Do you happen to have a modern, large thumb drive (modern = fast usb3) ?
If yes, look into rpi-clone utility. github.com/billw2/rpi-clone
It only clones used space, skipping temp files/directories created by the kernel/utilites and not needed across boots
If your PI4 is up to date firmware wise, rpiclone can give you a bootable thumbdrive which could be cloned back to the SD card if needed, or mounted after boot if need something off of it.
How much space is being used currently, a 'df -k' command in a shell will show percentages.