Kenshiro was wtitten 拳四郎 in the prototype Manga.Only once.
拳＝ken=fist
四＝shi=four ,fourth
郎=rou
郎　is usually used as the last part of man's name with numbers and simply means man.You can easily find Japanese men whose name end with ROU.

ＥＤＩＴ
As for Ryuken's brother,he is written 拳志郎.志=shi=belief.