How do I calculate the seasonal variation from a 4-point moving average?
As you have realised, with a three point or five point moving average, your data points are aligned with your original data. Well, the four point moving average would give you a trend line with data points aligned mid-way between the mid-points of your original data and so I think you should interpolate between the moving average data points. Since the moving average data points are evenly spaced either side of your original data, interpolation involves averaging successive points, so I agree with your proposal to take a further two point moving average as a means of calculating the trend. I would, however, refer to it as interpolation instead of as a further moving average.
