The 4431's minimum lux ratings should not be better than the 5231's. But since Dahua made the 4431 only for sale/use within China, AFAIK nobody's been able to find the "official" Dahua specs for this model. The entrepreneurs that resell the 4431 outside of China (after loading on a hacked firmware, as the official Dahua firmware for this model is in Chinese only) are known for putting their own specs in their ebay/AliExpress/etc listings.
Not saying this camera won't work for LPR, but from reading the reviews of it in this
thread, it doesn't seem to perform as well in low-light as the 5231. Makes sense .. it has a smaller image sensor combined with a higher MP lens ... that combination works against having better low-light performance than the Starlight models (like the 5231). It all comes down to how bright your scene will be... The darker it is, the better the 5231 should do vs. the 4431. Check out
@nayr's comments in post #12. That thread also has some comments from folks trying to use this 4431 as a LPR camera.
Remember that the inexpensive "Chinese-market only" models (like the 4431) require some special consideration. Normally you can not upgrade the firmware that comes on them. This may not seem like a big issue, but the latest 5231 firmware upgrade added a feature that I think might be useful for LPR -- the ability for the camera to switch between day/night profiles automatically, based on how bright the scene is. Also, if you're planning on recoding to a Dahua NVR, the latest NVR firmwares block the 4431s. Not dealbreakers for some, but definitely anyone considering one of these cameras should be aware of (IMO).