The size labeled "AE4" declines over time. It generally refers to the smallest coin denomination issued and AE3 to the next size up. In the era of Constantine, 17 mm is close to the borderline. Since these are usually 15 mm and rarely, if ever, reach 17 mm, they are AE4s in Sear's book. If you were talking about 5th century AE4s, they would be smaller and some of the larger (uncommon) types of that century can be called "AE3" when they only 15 mm because they are larger than the common 12 mm "AE4" types alongside them.