What is the difference between Samsung 4 files(Repairing firmware) and Single firmware file for flashing?
The main difference is that the single file doesn't Factory Reset the phone after flashing, but the 4 file firmware does factory reset and remove absolutely everything and make it like an out of the box phone.

The 4 files are called Binary Firmware which contains all the files in certain order:


Bootloader contains all the boot files
AP contains all the system and recovery files.
Modem/CP contains phone network and wifi software.
CSC contains PIT file and cache removing IMG which puts all the files in order and repairs any damage firmware has before. 


Thats why they are called repair files.