My son Jay & I did just that about 3 months ago.
Stripped forged lower receiver thru a local dealer and the complete PCR4 HBAR 20" upper kit with parts for the lower directly from OLY. It took about 2 hours to assemble the lower using the instructions on the AR15 archive site...
http://old.ar15.com/docs/assembly/lower
Assembly job was made easier using detent, pivot pin and roll pin punch tools from Brownells. We were impressed with the weapon right from the first... nice fit, finish and feel.
She has functioned and shot well with only one hitch, and that was human error re-assembling the hammer spring backwards after a disassembly/cleaning session, resulting in light primer strikes on 556 ammo next time at the range.
Uses USGI 20rd and 30rd surplus mags without problems, shoots Am Eagle and UMC 223 ammo without failure, and we are waiting for decent weather to test out 556 Federal XM193 and Win Q3131A, having corrected the hammer spring issue. Hits whatever we aim at.
Overall, Jay appears very pleased with the results. And OLY has a good reputation.
Good luck and good shooting.