.. 4gb is 4gigabyte. 8gb is 8 gigabytes. It is the storage capacity of the phone.. Why you don't know that? Your using one you don't even know.geez.lol. I prefer you buy 8gb because the difference is little. If i wer you, i go with 8gb. 4gb is somekind of phase out. And 8gb you can put more songs.