[AT&T Samsung Galaxy S 4 Q&A, Help & Troubleshootin] ItsJono:
Hello everyone!

My parents contacted me asking for help with some phone issues that I thought would be minor but this is now something I have to submit to you guys because I just don't know how to tackle this issue;

I've got an unmodified, completely stock AT&T Samsung Galaxy S4 that has been owned since launch. My parents were initially complaining about how the phone stopped charging and wouldn't turn on. I thought, "Easy, dead battery, it's been 3 years so how could it not? It might not be delivering enough voltage for the SoC..." but i'm not sure that's the full story which is why I want your guys' opinion since this place is a wealth of knowledge.

Things that are known:

When you charge the phone, the basic "battery charging indicator" that is shown when Android is not booted into or powered on freezes, or sometimes does not detect a battery so does not even turn on. Doing battery pulls seems to change how the phone responds. This is what my parents were experiencing. I thought, either dead battery, dead micro-USB cable, AC adapter, or charging circuit within the phone. Since this was obviously a big job, I took the phone back home for this weekend to play with and now this is what i'm experiencing.

So I got the phone to charge using one of my AC adapters for some reason, despite using the supposed broken charging cable to transfer data successfully without corruption using my own phone (LG G2) to confirm if it was just the cable. So now the SGS4 has a battery that is 100% charged, I turn on the phone and tried various methods to get the phone to boot successfully back into android. The first obvious one was to boot into safe mode using PWR, letting go of PWR, and then pressing VOL UP. It booted ONCE or TWICE successfully into safe mode into android, it shut off after 5 seconds being into the system due to low battery. This was BEFORE I was able to successfully charge the battery to 100% with my AC adapter but has failed to boot since.

Now since I've been failing to get into safe mode as when I boot the phone up, the phone freezes or has graphical glitches at the Samsung post/splash logo as seen here:

After some desperation, I considered using KIES/Odin to reflash, but mostly I tried going into download mode to see if the phone would even go into it by doing PWR + VOL DOWN. It did get into download mode but it froze/restarted forcing a battery pull and ever since then I keep getting occasional error messages such as "System software not authorized by AT&T has been found on your phone.", or this screen with a Samsung logo and an open padlock. I notice sometimes that when I'm at this open padlock screen, it sometimes begins to boot into the system but quickly freezes when it plays the boot jingle:

At any rate, I ordered a new battery but I fear this might be EMMC corruption or just something I don't quite understand. The phone has valuable information that I have to recover despite my parents CANCELLING a backup of the phone, they failed to heed my due diligence and are now freaking out about the data (photos, contacts, etc). I just need the phone to boot so I can make a nandroid backup, export the contacts, transfer media off the phone and wipe this thing and start anew. However i've already let them know that it might not be possible. The most important thing is to get this thing out of this soft-brick?

Please help because I have no idea where to steer this ship from here on out aside from waiting to receive that new battery and hope these instability issues are caused by low voltage. I had an SGS2 which had the crazy EMMC bug so I'm hoping its not EMMC corruption.

Thanks for your time everyone! I'll update this thread to reflect new information as I play around with the phone.

Edit: Here is a video, I go through a lot of the same information posted in this thread but here it is.

https://youtu.be/EXUKU68CyQQ