Help with unrooting

[James Bourne2]

Hi there. I am trying to unroot a samsung galaxy s4 for a friend. I am ok around a computer, but have never dealt with android (to the point of, until 3 days ago, I'd never heard of rooting).
Now, he purchased the samsung galaxy s4 off the internet. When you reset the phone to factory settings, the default language is Chinese or Japanese or something like that. So I have no idea where the phone originated from. I believe it is an international model (s4 lte gt-i9505).
Now I have watched a video on YouTube from androidrootz.com on how to do this. I have followed it step by step. But keep having issues. Also, not sure if I'm downloading the correct stock firmware for this device. The first firmware I downloaded and tried putting through Odin came up pretty much straightaway 'FAIL'. Since then I believe I have downloaded the correct stock firmware for this device, I9505XXUFNC4_I9505EVRFND1_I9505XXUFNC4_HOME.tar.md5.
How do I find out if this is the correct firmware? Also, I'm currently trying to put that firmware through Odin now, and it just seems stuck. Checking MD5 was successful, but now seems stuck on SetupConnection.
PLEASE HELP!
Ok, since posting this I have tried again. It has now got past the bit it was stuck on but is now stuck on another bit.
Odin messages read:
<ID:0/004> File analysis..
<ID:0/004> SetupConnection..
<ID:0/004> Initialzation..
<ID:0/004> Get PIT for mapping..
<ID:0/004> Firmware update start..
<ID:0/004> sbl1.mbn

And it just won't move from there.
Please help.

 

[Golfdriver97]

Files can be located at either Sammobile or Samsung updates .com. Look for the exact model number and if your friend has the box, the region code should be in the model number, typically at the end and is only 3 characters long.

Find the same file.

As for Odin, try different USB ports, cables, updating drivers, restarting the PC, running the program as Administrator.