select 4 rows, calculate stddev, take next 4 rows, calculate stddev

[hudo]

Hello,

I'd like to do some statistic calculations.
For example to select the first 4 values from emp.sal, calculate STDDEV and VARIANCE, MIN
and MAX from these values,
than take the next 4 values from emp.sal, calculate STDDEV,VARIANCE,MIN,MAX,
than take the next 4 values and so on..

How can this be done ?