Well, I think it becomes an etiquette question just if it goes beyond reasonable levels (like so many things in life). Four times in nine hours is still reasonable for me ...
However, recently, I also started complaining when my seat neighbour from ZRH to LCY wanted to go out for the 8th (!) time on this very short flight ... However, this didn't change her behaviour and in total she got out 11 times (not including the boarding and desembarkation) on this 90 minutes flight ...