Fortunately several great people have created helpful guides, reviews and documentation already. A huge THANK YOU guys!
First a series of excellent articles in Spanish from AUAmstrad website/group:
First look and adding ROMS to the M4 board on Amstrad noob (added 2021-04-23):
In Greek by Vincent GR:
Review by Chinnyvision in English:
In Germany from Jungsis corner:
Review and early setup by Am Strad in English:
And a guide in Spanish by Professor Retroman, that I linked to before:
Video for using the M4 board by Amstrad-noob.com in English (added 2021-04-23):
In Polish by RetroGralnia (added 2023-02-04):
Also Mr. DVG over at the cpcwiki forums compiled a collection of 1300 games tested and working with M4 board!
If I forgot or missed a useful resource, please do not hesitate to make a comment, so I can add it to this post.
Thanks again, all of you for your dedication, hard work and support!