When in Extend Mode, can I use my AirPort Extreme's WAN port as a LAN port? I have two AirPort Extremes. One has its WAN port connected to my modem, and the other as been configured to wirelessly extend the first, meaning none of the Ethernet ports on the back are connected. I plan to connect four devices to this second AirPort Extreme. I know I can use the three LAN ports on the back to connect devices, but can I use the WAN port?
Yes.

When an AirPort Extreme is configured to extend another AirPort Extreme wirelessly, the WAN port is not used for WAN, and this means it is automatically switched to be a LAN port, identical to the other three LAN ports already on the back of the unit. This was tested by myself with an Apple AirPort Extreme 802.11n 1st Generation, and this answer was posted with a computer which is connected to the WAN port of said router.