Refer to the exhibit.
Router6# show running-configuration
<output omitted>
interface FastEthernet0/0
ip address 172.16.6.6 255.255.255.0
ip nat outside
!
interface FastEthernet0/1
ip address 10.10.10.6 255.255.255.0
ip nat inside
!
interface Serial0/0
ip address 172.16.11.6 255.255.255.0
ip nat outside
!
ip nat pool test 172.16.11.70 172.16.11.71 prefix-length 24
ip nat inside source static 10.10.10.4 172.16.6.14
ip nat inside source pool test
Router7# show ip route
<output omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 4 subnets
C 172.16.11.0 is directly connected, Serial0/0
Router4 can ping Router5 (172.16.6.5), but not Router7 (172.16.11.7). There are no routing protocols running in any of the routers, and Router4 has Router6 as its default gateway. What can be done to address this problem?