How to take 4 variables and do a calculation on them then update to a constant stream of data

I have a constant stream of data coming into my program from an arduino. I want to graph the data but I need to create an acceptable variance in order to do that. I have created a user input to tell me their acceptable variance and then I'm stuck on what to do. My psuedocode for what I want to do is this:

        % take 4 values of data and sort them into an array
        % calculate the standard deviation of those 4 values
        % if standard deviation < acceptable variance
            % plot
        % else
            % 
        % end

I guess my question is, how do I take 4 values from my constant stream of data, do the calculation, plot that, then update the 4 values again...?

Thanks in advance