To upgrade to 4.x, uboot web/ GL.iNetUI both work, with nand-factory.img. (not sysupgrade.tar)
To downgrade to 3.x, only uboot web works, with factory release 3.x .img.
3.x doesn’t have luci installed, so when access http://192.168.8.1/cgi-bin/luci
/usr/lib/lua/luci/dispatcher.lua:255: No valid theme found
This error occurs.
To avoid any cached content in case of your browser,
disable cache or use incognito window should solve it.