I think you mean 4150 steel chrome-lined. RRA most likely contracts out the barrels from Wilson Arms, as do a lot of the manufacturers nowadays, and 4140 steel is what Wilson uses. RRA does have a few types of chrome lined barrels, I believe they offer the M4 and R4 (M4 heavey under handguards type) in chrome lined if you look around. According to the Adco Firearms site a chrome lined midlength will be available soon.
From my observations, it would appear that many manufacturers are going over to chrome lined due to consumer request for it. In the last year most everyone can get chrome lined M4 barrels now, and I bet it won't be long before 20" hbars come chrome lined. As for the stel type, I wouldn't hold your breath as probably 95% of people do not know that it is a different type of steel, much less what the difference is between 4140 and 4150.