pask

Maybe because you still use the RockPi4B DT and the LED pin is not the same as M4, you need to switch to to proper DT in /boot/armbianEnv.txt by adding this line :
fdtfile=rockchip/rk3399-nanopi-m4.dtb  

Right ! What I did to get eMMC running is I've frist removed it to get u-boot booting, stop it to get at the prompt, then I re-inserted the eMMC carefully while power still present , and boot the SDCard using "setenv devnum 1; run mmc_boot". When SD was booted and I've logged, I used "nand-sata-install" to install/clone the SDCard into eMMC ...

I have successfully compiled and run http://wiki.friendlyarm.com/wiki/index.php/Buildroot_for_RK3399 on a nanopi m4 v2.  I can confirm that uboot's makefile from these sources reports "version=2014".
My question is: how do I use the  working compiled uboot 2014 version with an armbian image? Is it possible to simply substitute some (compiled) file of the armbian image to do a first very raw try and see if the nanopi m4 v2 board boots and works?
 
Thank you and sorry for my very noob question
 
 

You could use balbes150 his N2 Buster image. This is with kernel 5.2. But almost everything works as it should.
Otherwise you could build a Buster image yourself if you've got an x86 pc.
https://docs.armbian.com/Developer-Guide_Build-Preparation/