From what I've read the 19Gen4 still seems to be hit or miss but the latest ones with the new RSA, extractor, ejector and trigger bar would be the ones to look for. I think the two fired casings in the box must be dated 11/2011 to get the updates already installed. You can check the number on the ejector to see if it's the recent one also, I think the old one was marked 336 and the new one is a longer #.
I got a 23Gen4 earlier this year and love it so expect you'd be real happy with its 9mm twin. The 40S&W Gen 4s haven't had many problems but I did have Glock send me the new RSA though. I haven't bothered to get the latest extractor, ejector and trigger bar since mine hasn't exhibited any problems.
If you do buy a Gen 4 with problems Glock will send you a FedEx label to ship it back for updates. They won't send you the parts except for the RSA.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek