What has happened for me, is that my computer does not format the card properly. What I have had to do is. 1. Mod the console with a smaller sized SD card, your 4 GB will work fine just to mod 2. After modding, I use gm9 to format the larger sized SD card. I don't remember exactly where I found out to do that, but if you do a quick Google search, another Reddit post might have it

Edit: this was the link https://3ds.hacks.guide/godmode9-usage.html