It’s something that is probably impossible to quantify – but even if each stroke of the cane was only 50% more painful than the stroke that came before it, stroke number 6 would be nearly 8 times as severe as stroke number 1, and overall, 6 strokes would equivalent in total pain to over 20 single stroke canings. On that scale, a 4 stroke caning would be more than 3 times more painful than a 2 stroke.
Most boys who I have discussed it with tend to believe that the pain doubles or triples with every stroke. If that’s the case, 6 strokes would be 60 times worse than 1 alone, and 4 strokes would carry 5 times the pain of 2. Personally, I think that’s probably in the ball park of being right, given what I recall of being caned (I would have taken 3 every day for a week to escape 6 once) and boys reactions.