Samsung Blocks Ability To Remap Galaxy S8's Bixby Button (zdnet.com)
A Samsung representative confirmed today via Twitter that the company has blocked the ability for users to remap the Bixby hardware button on the Galaxy S8. For soon-to-be Galaxy S8 owners, the news will come as a disappointment, especially since the Bixby voice assistant in English has been delayed and will not be fully functional when units starting shipping later this week. ZDNet reports: XDA Developers first reported a Galaxy S8 firmware update blocked the ability to remap the button to perform a variety of tasks. Before, the button could even be remapped to launch Google Assistant. It's not clear if Samsung will ever support remapping the button. A representative for Samsung tweeted: "Can't say it will never happen, but we won't officially support."
Samsung locks their bootloaders. In three years of ownership, I was never able to flash a custom ROM on my Galaxy S4, as no one was ever able to crack the bootloader. I just checked on the xda forums. Still not cracked to this day.
So, to answer your question, it's very hard.
I don't always use unix-like operating systems; but when I do, I prefer FreeBSD.
I'm not sure where you checked on the XDA forums, but it obviously wasn't the dedicated sections for the Galaxy S4 which is filled with custom roms, for several different variants of the S4:
https://forum.xda-developers.com/galaxy-s4
My wife's S4 has run Cyanogenmod for years, and now runs LinageOS.
It might be a Verizon S4... VZW takes bootloader-locking 'evil' to creative new heights (lows?).
Apparently, when the Note 4 came out, Verizon actually paid extra to Samsung for them to protect the Sprint version's bootloader the same way (Sprint itself was indifferent) just to make sure there wouldn't be another CDMA model with easy-to-unlock bootloader. From what I recall, the Verizon model of one of Samsung's earlier phones could be cracked by flashing a Sprint bootloader to the Verizon phone... it temporarily bricked the phone (or at least disabled the radio modem), but then you could unlock the easy-to-unlock Sprint-version bootloader & reflash it with a second bootloader that was basically a Sprint Android bootloader w/ripped Verizon radio modem firmware to give you a working, bootloader-unlocked Verizon phone. Verizon was determined to keep it from happening again.