I think there may have been a thread on the zebra size earlier - still interesting.
In my breeding colony (13) I think the fish are between 2.5-3" there may be one 3.25-3.5 but I have never seen a bigger one - come to think of it I have only seen one zebra that was not in my tank
I am often surprised to hear of zebras being 4" but maybe in the past, wildcaught fish that were bigger were introduced into the hobby. It seems that many plecos attain bigger size in the wild then in a tank and perhaps that is the same for the zebra.
I guess it depends a bit on how you measure the fish as well. You're not supposed to include the tail when sizing a fish are you? Imagine the size of lyretails like Adonis if you did (even though they get big enough!)