Hello Chris
If you’ll notice, the L0 interface on R4 with an IP address of 4.4.4.4/32 is in Area 0. This is not shown in the diagram with a colored ellipse, but is shown within the configuration of R4.
The Area 0 of an OSPF network can simply be applied to a single interface, even to a loopback interface, fulfilling the backbone area requirement of OSPF.
I hope this has been helpful!
Laz