Wireless Camera Design In A Finished House

Sice there is only 4 cameras Sonos BRIDGE units may do the trick. A separate 5Ghz WiFI network using Uniquity UniFi is another setup we have tried that works well.

What kind of framerates and resolutions are required?

Try to avoid the customer's home WiFi network if you can - because the cameras will slow down there WiFi network significantly.

Undisclosed, do you have a preference for IP cameras with integrated wireless or connecting IP cameras to a separate wireless bridge/router?

Obviously, you'll need to run wires for power. To the extent you do that, you could run network cable back inside somewhat?

In a huge house, a big risk is the wireless signal being attenuated by internal walls / objects. To that end, I second Bohan's suggestions above.

Undisclosed, try the CCtv camera over existing house electrical wiring like D-Link powerline adapter.