Even over a sample of 20 years:
3rd - 8 (40%)
4th - 2 (10%)
5th - 6 (30%)
6th - 4 (20%)
4th is the anomaly over this short period. As you've said you'd need to do this over a longer time frame.
If you think of the semi-finals, the winners of 3rd vs 6th have won the playoffs 12 times out of 20.
We've got 17pts from our last 10 games, Reading have got 18pts, Ipswich 15pts, Forest 10pts. It's going to be tough for any of these teams to get in the playoffs, it's an incredibly tough league (if anyone hadn't noticed over the past 2.75 seasons). To be fair our form is only fractionally worse...