Just wondering what serial number ranges to avoid when shopping for a used Glock 19 Gen 4

Thanks.

none. they all work. and if they don't, glock will fix them

There are no ghost in the closet. Gen 4's work just like all other Glocks do..Very, very well.  I have enjoyed three 19's since the Gen 4's first came out. Never had a problem. Same argument could be had for made in USA vs Austria.

No brand is above a poor example or internet case of being a lemon.

If you can get  a deal on a good used Gen 4 and pay 75% of blue label price...buy with confidence. If we said Blue Label price was $400 for easy math and someone wants to charge you more than Blue label price for a new model with 3 mags, then we might agree Glocks hold their value well for a reason. I would be perfectly happy buying a Gen 4 19 whenever it was made. My price point for negotiation starts at blue label price and works down. This goes against the grain of most people who paid full retail price or gun shops trying to make margin...

YMMV

Personally, I stay away from serials M through U in both Gen 3 and Gen 4 in 9mm, although I had a 21 Gen 4 that gave me grief, too.

All should work fine. Worst case you get an early one and need a different RSA, which Glock will provide.

And possibly an ejector, which Glock won't send you unless you are an armorer.  They are fairly inexpensive however.