Since 1979 when the 12 seed era began there have only been 67 semifinal matches that didn't include a top-4 seed.
The following brackets didn't have a top-4 seed in the semis at all:
1981 - 190 (5, 8, 11, US) [Placers were 8, US, 5, 11, US, 1, US, US]
2006 - 157 (5, 9, 10, 11) [Placers were 5, 10, 7, 1, 11, 9, 6, 8]
Fun Fact - Joe Johnson was in the semis against a non-top 4 seed in back-to-back years in 2005 (#8 vs. #5 Percival) & 2006
I think 1981 may take it.