Unfortunately you can't upgrade the original WS-2902 to firmware 4.x.x. You are stuck on on 3.x.x and probably 3.0.3 forever as it hasn't been updated in a very long time so that isn't a good sign. At one point Ambient allowed the original WS-2902 to get updated to 4.x.x firmware. Ambient removed the ability to do this quite some time ago. The reason they removed this ability is because it was a very sensitive process that sometimes resulted in a bricked station and that was causing warranty replacement issues. There really is no way around this.
The good news is that the update of the firmware was really only something that is done to the display console itself. Nothing changes with the outdoor sensor array. The reason this is good news is because if you want to upgrade your display console then you can get the newer and better WS-2000 display console (just the display console) and it will automatically pick up your outdoor sensor array. Essentially you can just by getting the new display console end up with a WS-2000 that supports all the new extra sensors, like lightning, water leak, soil moisture, PM2.5 air quaility...etc.
If a nicer display doesn't fancy you, but you want some extra features with a much lower investment upgrade cost then you could get the Ecowitt GW1000 and then this becomes your console and it will also support the extra sensors and it will also allow you to easily expand your station to support a bunch of 3rd party software; Cumulus MX, Meteobridge, Weather-Display, WeeWX and others. Some of these others are FOSHKplugin, and Personal Weather Tablet app, GW1000live and probably even more and new future ones for sure.
Or you could get both the WS-2000 and the GW1000.