Glock2366 is absolutely right about that! I forgot about making them single shot. It's just a block in the magwell or a mag that won't squish down. You just have to find a dealer who will do it.
You can also do a private party transfer. Like Glock2366 said, it can be someone who bought the Gen4 out of state. But also be sure that that person has a valid form of ID to show residence in CA. If a guy from NV drives a gun to CA to do a private party transfer, he will have wasted his time, because PPTs are a CA thing, and only for people with CA IDs or CDLs or Military ID with permanent station orders on hand to do it. Private party transfers are also consignment sales.
THere is also one other possibility: if you know a peace officer who is willing to sell you his or her off-roster handgun in a private party transfer, it's good. You just have to find a peace officer who wants to part with one ;-) There's the hard part :-) WHo wants to get rid of their Gen4 ????