um, i'm still confused as to what the TransFlash MicroSD is used for with the R4?
Let me elaborate on this though...
The micro SD will fit into the R4 cartridge and acts like a memory card, it is a memory card anyhow. You can download and save games and then play it as though you have the DS lite game itself like everyone else in here said before. You could carry more than 20 games on a 1 gig micro SD card so you don't have to carry little mini game cartridges around with you. But not only that, you can also use Moonshell, a Japanese program. Originally, way back when, it was only available in Japanese so most people were like

when it came to using it. But with the R4 English version, Moonshel is in English. When you get the gist of it, you could also change the GUI (skins/icons/etc.) for the R4 and moonshel which is pretty neat. If you search on youtube, people have really gone all out customizing it.
This is the best thing I've purchased so far for the DS. I just hope that Nintendo won't go crazy trying to shut everything down for card users. But then I am backing up every ROM I have and download heaps more

and maybe buying another R4 card just in case.