Not all of the 4GB is usable space. On my 15GB iPod only 13.9GB was usable. So, if its the same for the iPod mini then you'd only have 2.9GB, which would fill up quite quickly.
Hmm well considering i havent fully 'signed up' yet I.E bidding on ebay i guess i can go with the 20gb
Any ways, if y'all are going to do this, pleeeeeaseeee use my referral link! =(
http://www.freeiPods.com/default.aspx?referer=7335744
~Kujila