Hi this may seem like a dumb question to some of you guys but if my sd card is full do I need to buy another r4 card or can i swap different sd cards in the same r4 card Thanks
Why don't get an 16 GB card... You can use any other SD card on it, as long as it holds the correct firmware.
Or you can always delete games you never play off of the Micro SD card and leave the save files on there. Then, you can come back later and continue where you left off if you put the rom back on.
If you have a really tiny SD card (less than 1GB) then get a 2GB card (you can get bigger ones if you have an R4 SDHC). Or if you just have loads of games, you can't be playing them all, just delete a few that you aren't playing.