Instead of going with a complete copy (M4 carbine profile), I'd look at a mid length upper with a 16" barrel.
But if you're dead set on a close as you can copy -
http://www.bravocompanyusa.com/BCM-S...m4-14socom.htm
or
http://www.bravocompanyusa.com/BCM-M...-urg-m4-14.htm
You'll need to get it pinned with a flash hider that brings the overall length to 16".
Knight's RAS
http://www.bravocompanyusa.com/KAC-M...ac%2098064.htm
Palmetto State Armory is selling lowers right now for $50 +$7 shipping to your FFL.
http://www.palmettostatearmory.com/1560.php
Lower parts kit - All USA made and good quality parts.
http://www.palmettostatearmory.com/lower-parts-kits.php
You'll need a receiver extension (buffer tube) - you can also get this from palmetto state -
http://www.palmettostatearmory.com/stock-kits.php