I think it was around 1972 when the try became 4 points instead of 3. But yes, not sure when it was, but I am absolutely positive that the drop goal used to be 4 points. Also there used to be a 5th way to score unil around the 40s I think, it was something along the lines of you could catch a ball anywhere on the field from a kick, and a bit like calling the mark in your own 20, you would call for a dropped goal, after you had called you had the chance to kick a drop goal from where you stood, 1 or 2 steps run up/walk was allowed.
If you want specific dates or details on anything above I will have to get one of the rugby books I have out, will take me a while to find them though!