OK, vote totals for determining the final four to vote on.
1: 12
2: 7
3: 6
5: 11
6: 4
7: 2
8: 6
9: 2
10: 7
11: 3
13: 4
16: 2
17: 11
19: 1
26: 1
29: 2
35: 1
36: 5
37: 2
38: 2
39: 1
40: 13
41: 2
So, the final four are:
1 (Battle of Britain)
5 (Late-War 8th AF)
17 (Summer 1941 RAF raids)
40 (Hell Over the Hinterland, Battle of the Bulge air war)