I edited my previous post because I remembered things a little wrong. I remembered that /home/user would be mounted on a different partition from the root (/). /home/user is the directory where most of your data is saved. If you did mount it on a different partition, then your root partition would stay smaller. On a IT and limited memory however it is better to make one big root partition and use it. You probably can't configure the programs to use the "internal card" as storage, because it uses FAT filesystem instead of EXT2.

So from the top: Your root partition (/) is too small, you have a large "internal card" that you can't use to store email and other stuff because they are automatically stored under /home/user
The easiest solution is to partition the card so that your root is say 3,4 GB and "internal card" 0,5 GB. Then use the /home/user directory to store your files. It is the directory that has Audio clips, Documents, Images etc under it. If you need more storage, use the external card. You could also buy a 2 GB card dedicated as the system card and use your 4 GB in the external drive as a storage medium.

Sorry for the confusion