Hello.
The 4 FastEthernet ports you see at the back of your Cisco 877 are not L3 ports. In other words you cannot configure an IP address directly on them. You have to use VLANs for that matter. Use the following as a recommendation because it is not very clear what you are trying to achieve:
View Configured VLANs:
Router#show vlan-switch
Add VLAN:
Router#vlan database
Router(vlan)#vlan 2 name FIRST-TWO-SWITCHPORTS
Router(vlan)#vlan 3 name LAST-TWO-SWITCHPORTS
Router(vlan)#exit
Remove VLAN:
Router#vlan database
Router(vlan)#no vlan 2
Router(vlan)#exit
Assign Switchport(s) to Existing VLAN:
Interface FastEthernet0/0
switchport access vlan 2
Interface FastEthernet0/1
switchport access vlan 2
Interface FastEthernet0/2
switchport access vlan 3
Interface FastEthernet0/3
switchport access vlan 3
Assign IP address to VLAN:
Interface Vlan2
ip address 1.1.1.1 255.255.255.0
Hope this helps.