Yeah they are rare as you like so not surprised if you can't find other examples.
Prior to this everdrive lark. Quite a lot.
With this everdrive lark. Probably still quite a lot but it does mean there are other options available for RTC games.
As ever something is worth what someone else is willing to pay for it. Prior to everdrive's GBA offering I could well have seen it clear $100 USD. I could still see that but your market is a bit more limited.
Rundown for others.
Will require XP or XP VM to manage, might well require a battery change at this point.
4gb is 4 gigabit so 512 megabytes + 32 NOR.
Has RTC
Has a decent cheat engine
Has savestates. Not great/like an emulator but as good as they ever got on hardware really, probably better than any flash cart driven savestates on the DS.
Will come with a nice cradle/linker that allows you to dump GBA games and saves directly to your computer. (you will need to use the EZ1/2 EZClient program rather than EZManager though, you will also have to run EZMode beforehand).
Technically you can flash a DS loader to it and run some DS games there but it is not really worth it.
Compatibility wise is the same as
https://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 bar the RTC stuff, and there are some fixes in the client for some of the others but you might still want to use the later patches.
Among GBA era flash carts it is probably only the 8 gigabit version of this which is more desirable, include DS era GBA slot stuff and it is only the M3 and G6 line which will provide some real competition.