xize11
6
Plus I also want to recommend urgly if you come from version 3.xx to make a backup first and then download both factory image and the upgrade in that zip and use the factory image first via uboot without saving settings, you can copy manual config files via that backup file inside folder /etc/config/ which you made in luci, note that the network config might fail because things have changed since the dsa implementation of openwrt.
I have had alot of bricking when I was migrating from 3.x to 4.x and back, this is purely because its using a different kernel, if you already had 4.x flashed then you probably would be fine using the normal upgrade method. 