Excluding the 2 lowest and 2 highest values from a rolling STDEV

Currently I have written this formula which allows me to take calculate the standard deviation for a rolling 18 week range:

=STDEV(OFFSET(INDEX('[2011 NA Weekly.xlsx]Sheet1'!$B$5:$BA$5,,COUNT('[2011 NA Weekly.xlsx]Sheet1'!$B$5:$BA$5)),,-17,1,18))


However, I can't figure out how to exclude the two highest and two lowest values from this range. Any help is appreciated. Thanks!