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)