Welcome to the SeeMeCNC Community

I have the complete dual filament feed kit for a V3.
Want to use it on a V4.
Is the Y splitter and the EZR the same for the V4/SE300?

I realize there may be a 0.9 stepper/s on the V4.
Can the V3 EZR 1.8 stepper be used by changing a line in the DUET .config to 80(but uniquely/isolated from any other steppers running 0.9)?
Is that the same line of .config code?

OR,
for $10-15, I just order the proper stepper and don't mess?
I'm thinking through my question now, and realizing that maybe the filament feed rates might be too complicated?

The V4's mission would be to print plastic + removable/disolvable support material, and on very rare occasion I'd use plastic + some sainsmart/ninja flexible joinery.

Survey sez?

Yes.

Gracci Mac of the Knifage!
One of my favorite answers.

YES on the Duet .config mod for the one filament drive stepper as well?
I'm going to he Duet forum/instructions to learn about the .config, but might as well ask...

Yes, you can set the steps per millimeter at a different value for the extruder motor, that's how you would do the initial setup for extrusion rate. 80 is a starting point, but may have to be fine tuned depending upon the drive gear, etc.