[Sprint  Galaxy Note 4 Android Development] samep:
Same here, except I've moved on to another phone and haven't picked up my Note 4 in a good while. As I recall the basics, you need to check your bootloader version. Greater than OB7 limits Odin to the lollipop and marshmallow version less than QI5. QI5 limits Odin to that and up if there was anything higher.

That's really just speaking to Odin for full tar flash. Though I don't recall any significant advantage to using Odin for anything else but trying to recover a recoverable phone but beware the suggested limitations. Even then, a full tar is what is recommended anyway. I know there's lots of posts of using such and such to do what's desired, just know there's lots of consequences and later bootloader versions that locked against downgrading.

And while I'm recalling, don't forget to remove secure password, user data encryption and remove user account before flashing a different stock tar. As I recall the newer bootloaders tightened protections to the point that if a user followed a suggested lock when upgrading or restoring stock tar, anyone trying to flash without unlocking would semi brick it, even same user. This would require extra steps and research though I think, if flashing same stock version as it already had would just have Google ask user for original user and password. Without doing some extra digging, I'm short on keywords here. Sorry. Just fleshing out some recall for you is all.

As far as flashing older ROMs, I believe the trick was to flash the compatible kernel prior to boot. Don't trust my recall but I think it was flashing the kernel that matches the bootloader or noted as compatible with it; careful and don't mix custom kernels with CM. And data doesn't crossover from CM to stock versions or vice versa to the other either. Developers may refer to a bootloader version for best compatibility but it may work if you do it right.

As far as getting all flashy with your Note 4 again, just know that the flash memory is suspected as having a limited life possibly related to number of total flashes and updates and also could have been near dead or weak batteries too; flash and boot with app optimization puts the most stress on battery and flash memory, moreso than daily use and charge. Battery health, near or full charge and careful study is my recommendation. You might also start with a new battery. Order it while you're studying is my advice. Make sure the USB cable you use in Odin is a data cable as well. Remember, each flash is a risk as well as reward. Gee, you may want to look at your windows version and Odin version for compatibility too. I think there are recent posts about windows 10 use in later threads.

And there should be plenty of hints in each work you consider flashing so search those threads for keywords like bootloader and Odin and hopefully you don't break it. Many developers were keenly aware so the prerequisites should be helpful too as well as the install instructions. If Odin is recommended, remember your limitations. Others may confirm their bootloader version worked and how they installed; a good hint of what kernel they may have flashed prior to boot. I seem to recall one shortcut that may work is to make a fully stock backup with TWRP to restore prior to new ROM.

There is a TWRP thread. I'd suggest the latest one known to be compatible with the Note 4. Links in OP or near end should point you latest compatible version or even suggest which is best. As I recall, the latest ones mentioned worked just fine for me.

Backup your current ROM and Odin restore of full stock tar may be the place to start. Just make sure you're flashing the tar that best suits your current bootloader version. And make a stock backup after you're booted and have network connection and have TWRP.

As far as modem, I don't think there's any really any benefit of one over the other but I think I read it should closely match the bootloader anyway. IIRC. So staying with full tar that is compatible for flashing with your bootloader and a kernel compatible with the bootloader is what you need. I think that's the hint I referred to earlier, anyway.

Maybe it comes back to you as you review your possible flashes. Study should confirm that anyway. The old adage, measure twice, cut once applies if not moreso. It's all in the prep before execution. Hopefully the result is success.