Strange things are hapening with my micro sd, can you help me?

I have a 4Gig micro sd for an r4 (thats a game card 4 ds). My computer can read it, but the problem is: my r4 can't. I tried formating, but it wont work.

What I did was this:
I wanted a gba-emulator for my r4, but when I putted it on my r4 and I booted it, it said that he had to 'Drop' something. When it was done, I had to reboot the thing and it didn't work. Like I said before, my computer can still read it, but my r4 can't. Somme files have been ereased and the micro sd is "full", but it is just filled for 3.81 Gig. I can't add till 4 Gig, but I can delete things and add new things (whitch is useless if I can't read it with my r4).
Does anyone have an idea how I can fix it?
There is just one problem: I am 16, so I have exames and I probabely can't check this untill the 23st of July.

---Sorry if there are wrong words of sentenses; I speak Dutch and I am not realy good in English.---

Pieter-Jan (You can address me as Pidgey/PJ as well)
June 2009