I had (2) G4 23's a while back & they were both fantastic. Had around 1k through each. I've since traded them off, but I wouldn't hesitate to get another.
Ran fine as a 40. But also only ever used full power duty ammo, winchester ranger-t 180 gr I believe.
I had a conversion barrel on my m&p 40 and understand the breach face differences, but that gun still ejected brass reliably so I was surprised to see it launched forward today.
Will try some AE (which is our issued training ammo) and duty ammo next go around.
Is this the same gun you mentioned in the other thread with Trigger issues? That could be part of it as well.
If it continues to malfunction with good ammo and otherwise in spec parts you may want to contact lonewolf. JHC and KevinB have reported improved extraction/ Ejection in some guns with KKM replacement barrels so theoretically, the reverse could be true as well, a barrel a bit out of spec could cause those issues.
Since you've got both a 19 and 23 (and assuming they're both either gen 3 or gen 4) try your 9mm ejector and/or 9mm Apex extractor and see if that helps. Sometimes it does, sometimes it doesn't. Sometimes it does when the gun gets some miles on it. A few of the locals have not just gone down the 9/40 parts swapping road, they've paved it and put up signs. More trouble than it's worth to me, but it seems to satisfy them.
Anyway, that's a $0 experiment and if the ejector works you're out like $8. If it's the extractor that's a bit more spendy, but since you're already into it for a barrel why not a $60 apex extractor?
I have a 23 I converted to 9 with the LW barrel, it's a gen 3 but it ejects fine. I installed a 9mm ejector (trigger housing w/ ejector), 9mm extractor, and spring loaded bearing lci 9. I used all OEM parts and they were only a little over $20 for all of them. If you cannot order parts from Glock directly you can get all the parts on brownells (they ship faster than Glock so I'd go with them anyways). Ejection with crappy reloads is weak but it doesn't malfunction, but with any halfway decent factory ammo ejection is fine. May or may not fix your problem but it's a fairly minor expense to get the 9mm specific parts.
I don't shoot 40 anymore but I would advise against even trying to see if that would work. I would only install the 9mm parts if you are planning on converting it permanently to 9mm. If you are looking to switch back and forth between 9 and 40 I would just deal with the erratic ejection as switching the ejector, extractor, and spring loaded bearing each time you wanted to switch caliber would be annoying.