I am actually facing the same problem, impossible to find M3.5 screws and nutlocks, so thinking about using M4 like you mentioned but am i not going to face a problem where you will have to place M4 nutlocks in the nutlock trap and tool mount of the Z assembly when its designed to receive M3.5 ???
Also do M4 screws will go through M3.5 holes of the croners assembly ?
Thanks on advance for your input on this and your devotion to this project :)