I spent hours on this one and finally worked it out. I have four kids and the ordeal started when I ordered two R4 cards and they turned out to be different, so I found out lots.
Odds are, you have bought a clone R4 card, not the genuine thing. If it is genuine, you can download the latest software from
www.r4ds.cn. If that doesn't work, you more than likely have a clone.
The genuine R4 card uses encrypted software, so you will never get it to work with the genunine download. Just a well all the clone manufacturers have clone sites with clone software, the trick is to know which one you have and what can work. Here is what I discovered.
Turn the R$ card over and look at green printed circuit board just above the electronic contacts. You may see some white printing.
One of my cards says "R4-SDHC". This means that the R4 card can take a high capacity SD card. There are any number of versions of the software available, try
www.r4sdhc.com. At the time this was written, version 1.32 worked just fine for my card.
The other card says "ROHS CARD-7A". This is not a high capacity card and no software will work with an SD card with higher than 2GB memory. So if you are trying with a 4GB card, like I was, it won't work. For this one, I switched to a 2GB Micro SD card and downloaded "r4team v 1.19" from
www.linfoxdomain.com and it worked fine.
If the above software downloads don't work, there are a stack available on
www.linfoxdomain.com under the Nintendo DS tab, so go with trial and error, and you should eventually get there. Note that It took a few goes with the first card before I found v 1.32 and got there.
So good luck, I feel like I know so much more now.