What are some good NAS devices that support Time Machine over SMB and have at least 4 TB?
Synology NAS products are often recommended by users on the forum. Basically, you build your own NAS according to your needs.
https://www.synology.com/en-us/support/nas_selector
I'm using a Google Wifi Mesh network but it's not compatible with my Airport Time Capsule
My neighbor's Time Capsule.....which I helped him set up......works just fine with his Google mesh network, so not sure what you mean by "not compatible". I suppose it is possible that Google offers different models with different features, etc. , any mesh router that I've seen provides an Ethernet port to allow the Time Capsule or another Ethernet device to connect to the network.