Daniel Dib writes on his blog:
The Cisco ISR G2 routers have been around for a while now. Roughly a year ago, Cisco released the Cisco 4451-X router which was the first ISR running IOS-XE. Cisco has now added new routers to the 4000 family, which means that the ISR G2 family will eventually go away. Don’t panic though! That will not happen for a while but if you are looking to buy new ISR routers, then take a look at the new 4000 family.
One great thing about the new ISR 4000 routers is that they support upgrading of the bandwidth capacity by buying a license. That means that you can keep the same router for a longer time and grow into it, rather than doing a complete replacement as your demand for bandwidth increases. The new models are ISR 4321, 4331, 4351 and 4431.
Cisco showed off the new ISR 4000 series at Interop this year. Check out our video coverage of the unveiling at Tech Field Day Extra!
Read more at: Cisco Adds New Routers In the ISR 4000 Family