Napster is in fact a commercial transaction based on barter and gift. No only is the basis of the transaction familial versus commercial, the scope of the transaction is far smaller. The difference in 1 and 4 really matter. This closing of a market to the copyright holder clearly impacts the market and value of the work. While there is anecdotal evidence both way as to the impact of Napster on CD sales the presence of a free substitute of sold good cannot reasonably be taken to have only de minimis impact.
Most people only share their music with a small circle of friends.
The impact on this can easily be argued as non-commericial and de minimus. Napster trading cannot be considered fair use.