the obivous solution first :)

egrep -B4 /foo/ file
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for perl, maybe like (not sure you want to slurp .. this just keeps an array of the last N lines):

# put the line into the cache/queue;  shorten the array if it's more t
+han we want; print the array if current line mataches.
perl -ne 'push @lines, $_;  shift @lines if scalar(@lines)>4; print jo
+in "", @lines if /foo/' file

# if you don't want the current line printed as one of the 4, just rea
+rrange a little:
perl -ne 'print join "", @lines if /foo/; push @lines, $_;  shift @lin
+es if scalar(@lines)>4' file
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or, using Tie::File:

use Tie::File;
use Fcntl 'O_RDONLY';
my @array;
tie @array, 'Tie::File', $ARGV[0], mode => O_RDONLY or die;
foreach my $i ( 0 .. $#array ){
  # no, doesn't handle index edge case of $i < 4
  print join "", @array[$i-4,$i-1] if $array[$i] =~ /foo/;
}
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