The first topic already covers what works and doesn't.
The majority of R4 clones no longer work on the latest 3DS firmware. If you don't see yours listed in the "what to get" section, it pretty much means your getting is likely a dead cart.
Some newer clones put the firmware that it works on in the description. In many cases and examples from the board here, most R4 clones are only good for the single firmware they are listed for. It also means you will likely be buying a new R4 clone to replace the one you got every time you upgrade your 3DS.