If you have a pistol marked Lower and put a m4 stock (or any stock) on it it now is considered a rifle, If the barrel is under 16" then it is a "short barrel rifle" and in most places it requires a tax stamp or license to own. I have built to many AR's to count and I stay away from building pistols. For one they are not the most reliable of the AR's but once you get them tuned they are usually fine. The rifles on the other hand are plug and play for lack of better terms. The parts are mil-spec and I have not run into any interchange issues from any parts suppliers. as far as the law goes when you purchase the lower receiver you have purchased a "gun" not just a receiver. so building your own from parts is no issue at all as long as you use parts that are legal for the application, Pistol receivers must have a buffer tube only and it can not have provisions to slide a stock on it (but I have seen several people do this). Have fun and enjoy building it yourself. the best upgrade you can do whild building it is a RRA match trigger they are not expensive and well worth installing.
CPL holder since 1991, NRA Lifetime Member, DU Member, NICET certified
"For those who would give up essential liberty to purchase a little temporary safety, deserve neither Liberty nor safety.—Ben Franklin.”