https://openwrt.org/toh/linksys/ea4500
https://wikidevi.com/wiki/Linksys_E4200_v2
When flashing my E4200 according to these instructions i cme up with an issue / errro telling me the file i was tring to upload was incorrect format. No matter how many times i tried to update the firmware from within the UI i had the same error.
I finally decided to stick with the Cisco / Linksys software and noticed there was a newevr Linksys firmware update available. I did the Linksys update and had now a flashy new bright good looking UI.
I had a though "may i try the openwrt firmware update just one more time?"
I tried the openwrt frimware update and it worked!
So here is my expreience and advise to install/ flash OPENWRT onto a Linksys E4200 /E4500 router.
1) Ensure you are on the latest Linksys Firmware version either 2.1.41 or better (2.1.41 was the the firmware i used on my E4200)
2) Do the firmware update from within the Linksys UI and use openwrt-kirkwood-linksys_viper-squashfs-factory.bin
GoodLuck.
Further Info:-
Switching Lynksys routers from partition 1 to partition 2 and visa versa.
SSH method
You can recover the Linksys stock firmware by using the basic SSH in PuTTy (for Windows®) and use the following commands:
Determining your router’s current partition
/usr/sbin/fw_printenv -n boot_part
Booting your router
For Primary Partition: /usr/sbin/fw_setenv boot_part 1 && reboot
For Secondary Partition: /usr/sbin/fw_setenv boot_part 2 && reboot
For Secondary Partition: /usr/sbin/fw_setenv boot_part 2 && reboot