Hello!
Will there be the 4.x new firmware for these AR-750 Creta and AR-750 Slate routers?
Thank you
The latest 4.x pre-release firmware for AR750 / AR750S(Slate) can be found on GL-iNET’s github repository, however I never try it, you can give it a shot, good luck. 
AR750S (4.0.0_beta5): https://github.com/gl-inet/gl-infra-builder/releases/download/v4.0.0_beta5/ar750s.tar.gz
AR750(4.0.0_alpha6): https://github.com/gl-inet/gl-infra-builder/releases/download/v4.0.0_alpha6/ar750.tar.gz
Release note:
Or you just can’t wait you can try the vanilla OpenWRT flavors based on OpenWRT 21.02.3
AR750: OpenWrt Firmware Selector
AR750S NOR: OpenWrt Firmware Selector
AR750S NOR/NAND: OpenWrt Firmware Selector
Thank you very much !
But there is not the GL UI there, some functions are very useful in UI
Ya, that’s the trade-off you need to justify.
If you need friendly GL UI, you stay on 4.x firmware(if there is any) or you step forward and get familiar with vanilla OpenWRT for the latest version. 
Will 4.x be released on mt300n-v2?