[Galaxy S 4 Q&A, Help & Troubleshooting] Pulse654321:
For those that are still searching for the answer here is what I did. Links provided for ease.

This method will delete all user data.

This basically is a last resort type scenario so please follow it with caution.

Backup what you can, while you can. Plenty of tutorials online for that.

Download the ROM I9505XXUEMJ7, can be found at SamMobile but slow download. I alternatively used a pre-rooted one just for testing as I had no intention of keeping the Samsung ROM. You can find a copy here.

Download Odin 3.07 and 1.85 (I had problems using some files with them so had to keep swapping)

Make sure you have Samsung USB Drivers installed. (Download here)

Download I9505XXUEMJ7 WiFi FIX, here.

Optional Downloads:

Download CF AutoRoot 1.65, here.

Download CWM 6.0.4.4 .TAR files, see this post.

Now, If you're coming from a Stock ROM do this:

Go into Settings > Developer Options, and enable "USB Debugging". If you don't have that, you need to go Settings > About, scroll down to "Builder Number" and tap it 5-10 until it says "You are now a developer" and the Options should now be visible.

Put your phone into Download mode. (Power off/Remove battery. Then hold Vol Down, Power and Home button until it vibrates. Then continue if asked.)

Connect your phone to your computer via USB cable (if you've only just installed the USB drivers you may need to wait a while for it to recognise the device, you may also have to disconnect/re-connect the USB cable after it's finished loading the drivers.)

Open Odin 1.85 and load the downloaded ROM under the PDA Section (You don't need to change anything else)

Once Odin has added this device, should show "<ID:0/003> Added!!" or similar, click start.

When it's completed it should say "PASS" in green at the top. If it doesn't repeat steps 3 & 4 but with Odin 3.07.

Let it boot up normally, if it gets stuck on Samsung logo (Mine did), you'll just need to clear the user data & cache.

To do that you need to enter Recovery (Same procedure as download mode but Vol Up, not Down.)

Scroll down to "Wipe data/factory reset" and then scroll down to "Yes".

Then select "Reboot system now", it will restart straight away

Once it has finally loaded up select your language and then the next screen should be Wifi setup, if your wifi is working then you're done.

If you Wifi isn't working and won't turn on and is stuck, then continue the setup for the phone as you wish, I skipped everything and didn't enter any accounts as I didn't wish to keep it.

Once you're at the Homescreen, turn the phone off and put it in download mode again.

Open Odin 1.85 and select the Wifi Fix file under PDA. This time untick Auto-Reboot and then click Start once Odin sees the phone.

When it's completed it should say "PASS" in green at the top. If it doesn't repeat step 9 but with Odin 3.07.

Note: Most tutorials do state to add this under Phone, but I found that it kept failing and didn't work.

Turn your phone off and go back into download mode. (Again, I just pulled the battery out)

Close Odin and follow step 9 again. But you don't need to untick Auto-Reboot. This is to ensure that the Modem has been flash correctly.

When the phone has loaded again, try the Wifi out, hopefully this time it should be working. That's all I did after days and days of testing, and now it's working flawlessly.

If you then wish to install a custom ROM again, you are more than welcome to, quick guide:

Enable "USB Debugging", as above (Step 1).

Put your phone into download mode.

Open Odin 1.85, under PDA select the CF AutoRoot 1.65 file found at the top of this post.

Click start, let it restart and install SuperUser.

Once it's booted up, turn it off and put it back into Download mode.

Open Odin 1.85 and select the CWM 6.0.4.4 file and click Start.

When the phone has booted back up, you should now have working Wifi, a Rooted phone ready for exploitation and have Custom Recovery menu ready for you to install your new choice of ROMs.

Took me a long time to write this up, but if it helps just one person, then it will have been worth it. Enjoy!

Edit: I should also say a massive Thank You to all the developers out there that helped me get to this point by providing me with the Utilities, ROMs, Fixed Modems, Root and CWM.