[One (M9) Q&A, Help & Troubleshooting] Flippy498:
Please don't forget that this is a forum and not a support-hotline/-chat where people get payed for replying immediately. Most of us are doing this as their hobby and aren't able to be online 24/7. It might take some time till you get a reply. Especially if you post in a section that shows almost no activity aside from some new questions being asked. Most (former) users have already moved on to other devices due to the age of the M9 and many devs left the development scene due to the end users on xda that became more and more toxic and demanding during the last couple of years.

You actually don't need firmware 4.x. Firmware 3.x is compatible with the latest custom roms, as well. They only run "better" on firmware 4.x.

HTC is a bit lazy in regard to updating their webpage. You're always only able to download the latest RUU. However, you don't mention which variant you own. There is more than just one non-dev edition variant. Nevermind, you mentioned it in one if your following posts.

Firmware 4.x is the latest version for all SKUs/variants and there won't be any further updates since the M9 reached the EOL stage almost a year ago.

Is that a typo? I only know about firmware 4.27.531.6 for the T-Mobile US variant.

It's possible to break the OTA-update function by modifying the system via a root file explorer, as well.

Is that another typo? I'd have expected error 132 since firmware 3.x and 4.x are using different encryption keys. You can't update using a RUU. You need to use the software update function of the system settings.

The "database" is still stickied (in not only one but two subforums of the M9 section - the general and the q&a section). Therefore, I'm a bit surprised that you weren't able to find what you're looking for. In addition the thread I'm talking about is not only a database, it even contains all information you would have needed for solving your issues, already.

If the RUU is older than your current firmware then you won't be able to use it.

Error 155 means that you either try to flash a RUU that is too old or made for a different variant than the one you own. The key combination you're looking for is the power button plus volume down up and you need to keep them pressed for about 30 seconds. HTC changed that on one of the devices that came out before the M9. Letting the battery die is a bad idea, though. You'll damage it this way.

No you don't. I can't discourage people enough from relocking this phone's bootloader if the phone is S-ON and not 100% stock and you don't have a RUU at hand that you already used successfully on your phone. HTC's security checks will prevent the device from booting if anything is modified (e.g. TWP being installed) while the bootloader is locked or relocked. They don't differentiate between "broken" and "custom" files and think that the modified software is broken. Therefore, the checks are trying to prevent the hardware from getting damaged by preventing the device from booting. In addition, if OEM Unlock isn't activated in the developer settings of the currently installed rom then there is no way to re-unlock the phone. That's an FRP (factory reset protection) Google introduced with android 5.1 (which equals firmware 2.x of the M9).