$ cat ash.txt
1 2 3 45 32
2 3 5 36 87
5 8 3 96 23
8 9 6 24 56

for 4th column: 

$ awk 'min=="" || $4 < min {min=$4} END{ print min}' ash.txt
24

$ awk 'max=="" || $4 > max {max=$4} END{ print max}' ash.txt
96

or

$ awk '
min=="" {
min=max=$4
}
{
if ($4 > max) {max = $4};
if ($4 < min) {min = $4};
}
END {
print "minimum:" min;
print "maximum:" max;
}
' ash.txt

minimum:24
maximum:96