[Verizon Samsung Galaxy S 4] Atomicmuffin:
First...my sympathies on losing the first MDK to VZW's sleazy update practices. 'Tis a sad day for all.

Unfortunately, my investigations into the jfltevzw bootloader have been abruptly halted, as my S4 took a tumble the other day and obliterated the LCD. I'm debating on whether to order another replacement LCD assembly (i've replaced it once already) or just to let it die. Regardless, any replacement ass'y would probably ship from China just like last time...meaning it'd be many weeks before it arrived. :/

Anyway, so you're looking for tips on handling your new MDK unit coming?

Note:

For any apps/files/etc needed on your MDK unit, I would recommend using an external SDCard at first, at least until the device is in a state where there won't be any surprise ninja-updates by VZW. (i.e download stuff on your non-MDK S4, put it on external SDCard, pop it into your MDK, etc.). Otherwise, ADB works well too.

Rooting

So, first thing you're gonna wanna do upon receiving your device is disable Wifi and Mobile Data (if applicable) to avoid any accidental updates. Then you're gonna wanna root (obviously). With MDK being on an old kernel, it should be vulnerable to Towelroot, HOWEVER don't quote me on that. But go ahead and try it out. https://towelroot.com. If it's a no-go, there are lots of threads here on how to root. Make sure you don't forget to install your preferred SU management app (i.e SuperSU).

Stopping Updates

Once you've achieved root, i'd recommend immediately installing an app with system-app-freezing capabilities such as TitaniumBackup or 3C Toolbox. Locate the "System Update" app and freeze it. Following this, you can safely enable and connect to Wifi.

Install Custom Recovery

Once system updates have been safely frozen (or uninstalled) and you've connected to Wifi, you should be able to just install the TWRP Manager app from the play store and install that way. If not..

Only use this method if TWRP Manager does not work....Start by installing a terminal emulator (on the MDK device). Afterwards, on your MDK device, navigate to https://dl.twrp.me/jfltevzw/ and download the latest TWRP image file (twrp-3.0.2-0-jfltevzw.img). REMEMBER THE PATH WHERE YOU SAVE IT. (i.e /storage/sdcard0/twrp-3.0.2-0-jfltevzw.img)

Note: MAKE ABSOLUTELY SURE you're really on MDK, or this next step could be deadly(ish). Also, be VERY meticulous and type everything EXACTLY as shown.

Open your terminal emulator and do the following, accepting root-access where necessary. Also note, your username may not be "u0_a207." This doesn't matter.

u0_a207@jfltevzw:/ $ su
root@jfltevzw:/ # dd if=/storage/sdcard0/twrp-3.0.2-0-jfltevzw.img of=/dev/block/platform/msm_sdcc.1/by-name/recovery bs=4096
root@jfltevzw:/ # reboot recovery

Replace "/storage/sdcard0/twrp-3.0.2-0-jfltevze.img" with the path to the TWRP image you downloaded. MAKE SURE IT'S CORRECT.

Epilogue

Okay, you're on your own! Remember, read read read! And as a rule of thumb, try to keep these in mind:

Absolute first step after getting TWRP, BACKUP! Use this to return to stock in the future.

Double-Triple check ANYTHING that requires ODIN to flash. Never allow any firmware package to write to SBL1-3, or aboot. Except if returned to stock MDK. (See below)

If you need to undo everything and go back to stock and you cannot use your TWRP backup made in step-1, do NOT use any Verizon tools. Use ODIN, and use ONLY the stock MDK firmware package.

Once TWRP is installed, you typically won't need to flash your Recovery partition, so make sure you know what you're flashing if something wants to overwrite it.

That should do'er. Also, check out the post I made like two pages ago: http://xdaforums.com/showpost.php?p=67194262&postcount=2586

Remember, if you aren't sure, ASK. If something doesn't seem right, or feel right, STOP and SEARCH or ASK. You are welcome to PM me if you have questions (in fact, I welcome it. Because as you know, once the bootloader on an S4 is locked, there's no turning back...yet.) I may not reply super fast, but I will reply when time permits.

And of course, while I'm all for helping people out, I must paste the obligatory "I'm not responsible for your actions, devices, nor anything in-between." Have fun, and safe flashing!