Just to show a different way of working …
I dislike the concept of direct cell referencing (A1 etc) so I use Names and, as I prefer not to over-use relative referencing, I tend to use array formulae.
Given a column array called 'array' with a heading called 'heading' it is possible to define a record number 'recordNumber' using the Named formula
{= ROW(Array) - ROW(Heading)}
Then your formula can be defined to refer to
{= CEILING( recordNumber, 4 ) / 4}}
So far this 'formula' is just a recipe for a calculation. It might make sense to enter it into the array
{= formula}
to evaluate it but then again, you could just use it in another formula, e.g.
= STDEV.P(formula)
which, for an array of length 20, would give √2.