Hello Steph,
YES...the Panzer Arms M4 with a fixed stock, like the PAM4CSBMFSS model you specified, is legal in California. Because the stock is fixed, the pistol grip alone does not make the firearm an illegal assault weapon.
The shotgun has a fixed tube magazine with a 5+1 capacity. Since it does not have a detachable magazine, it does not fall under the state's prohibition on certain semi-automatic shotguns with removable magazines.
Hope the above info helps Steph 🙏