I got the chance to install a new Cisco ISR 4431 today, and the best surprise (besides the hugely upgraded throughput) is that the onboard router interfaces now support LACP! Before, you had to resort to getting a switch HWIC module, or just using "mode on" channel-groups. Since the 4431 has four gig interfaces, you can use LACP for two WAN interfaces, and use LACP for two LAN interfaces using the interface command
channel-group 1 mode active (use any channel group number you need), and then configure the port-channel interface.
interface GigabitEthernet0/0/0 no ip address media-type rj45 negotiation auto channel-group 1 mode active ! interface GigabitEthernet0/0/1 no ip address media-type rj45 negotiation auto channel-group 1 mode active ! interface Port-channel1 ip address 10.0.0.1 255.255.255.0 no ip redirects no ip proxy-arp load-interval 30 no negotiation auto
show lacp internal
show lacp neighbor