In June of 2013 Cisco released the newest member of the ISR family, the ISR4451-X router (Cisco – blog post). At that time it was the only model in the ISR44xx line, but today it now has some new siblings. Today, October 2, 2014, Cisco has introduced some companious, namely the ISR 4321, 4331, 4351, and 4431.
The below graphic from Cisco that shows the new line and the performance characteristcs of each. You will note, for example, that the ISR4321 says 50–100 Mbps. The base version of the device will support 50 Mbps, but to move to 100 Mbps on that box you just need a software license. This will allow a network to grow with it needs without having to replace hardware, you will just have to add a license file. Back in the ISR G1/G2 days we need to sometime replace hardwre in order to gain additional performance, but that is not the case with the ISR 4000 series. Much of that comes thanks to Cisco using IOS-XE on these devices. A quick blurb IOS-XE from the previous link is :