They are both on the same 1/2.8" sensor, so the 4MP requires double the light of the 2MP camera.
The 4MP during the daytime can allow for a little more detail, along with a little more digital zoom than the 2MP. But at night time, the 2MP has a far better range and clarity due to needing half the light of the 4MP.
In most instances, you want to get a camera that will perform at your location for the worse situation, which for most of us is at night when it is dark and there is little to no light. If a camera performs at night, it is easier to tweak settings to make it work during the day than it is the other way around.
So unless you have stadium quality lighting at night or do not care about night time performance, the 2MP will be the more versatile option.
Other than 4MP versus 2MP, the camera is exactly the same. Same size, same sensor, same optics, same GUI, etc.
Now when it comes down to Andy only has the 4MP left and your option is $400ish or $800ish for a camera with autotrack, then I'd go for the 49425 LOL.
Absolutely zero concerns. Many of my cameras are on older firmware.
A common theme around here is don't fix what ain't broke. If the unit is working and meets your needs, in many instances an update breaks what you had working and provides you with something you didn't need or bricks the camera. In most instances, updates are simply security vulnerability patches (usually years after the breach was found), but since we do not give our cameras internet access, the update is useless to us.
Here are issues I have seen people report here where they were upgrading just for the sake of upgrading:
A Dahua Z12E that someone updated and then constantly reboots comes to mind.
The Dahua 49225 and 49425 PTZ that loses autotracking with an update come to mind,
The Hikvision DS-2DEA425IW-DW PTZ that loses autotracking with an update comes to mind.
A Hikvision ANPR camera losing half the FPS and loses the ability to read US plates - those are big deals to have happen.
A Hikvision ANPR DS-2CD4A26FWD camera that lost all ability to read plates - kinda makes the camera useless.
A Hikvision camera that the user lost ability to control the LED light function at night.
A Hikvision wifi camera that loses the ability to use wifi after a firmware update.
Countless other instances where the camera or NVR simply bricked and became useless.
Countless examples where the camera or NVR went into Chinese.
Don't do it unless it is fixing a problem you are experiencing or adds a feature you really need.
Isolate the cameras from the internet and no reason to have to update...If a camera is working perfectly for my needs, why attempt to update it and brick it or lose a feature I use?