When the seller states that the camera firmware cannot be upgraded that's most likely because it's a chinese market camera, which the 4431 series is. Is it locked in stone? Yes and no. The cameras have a hardware region ID that the firmware checks, and the standard international firmware either doesn't work or won't do english. Whether the camera bricks or reverts to chinese for the UI, I thankfully have not found out. Going back over a year, the hacked cameras shipped with firmware version 2.420.0000.21R. Image-wise it worked well, the IVS features worked, but there were some interface compatibility problems with a dahua NVR that made their use a bit of a pain. The hacked cameras now generally ship with the current 2.420.0000.22R firmware (hacked) that has fixed the NVR compatibility problems, and in my experience is indistinguishable from the operation of a genuine international camera. There's a thread about
tools for modifying the firmware here
Dahua Firmware Mod Kit + Modded Dahua Firmware. The 2.420.0000.22R firmware has been modified by industrious people and is available to update 4431-series and other chinese market cameras. If that version works well enough for you, you're set. If there's a future version you want to upgrade to, you have to either hack it yourself or count on somebody else to do it and make it available.
Aside from the firmware issues, many of the chinese market cameras are marketed as "starlight". When you read about starlight cameras on the forum it is referring to 5xxx or higher series cameras, that have larger image sensors than the 4xxx series cameras. I don't know the formal definition of "starlight", but I believe the term is being misused with the 4xxx cameras. The 4xxx cameras have good images and are IMO a great value, but they definitely do not have the low light sensitivity of the starlight cameras discussed on the forum.