Most likely 4 available seats. There are 4 available "P" fares, but they will sell you a F or A fare as well. Someone else can probably explain it better, but basically, if there is X number of a discount fare code, there will be at least that number in full fare seats, and any fare code in between.

Edit: to add that Globaliser is correct, in that the correlation between fare bucket availability and actual number of empty seats is often weak.