I think I figured it out.
Cura 4.0.0 now has a new feature where you can select different number of cooling fans.
I looked at the gcode exported from cura 3.4.1 and the gcode from cura 4.0.0. I noticed the M106 command was exported as M106 S255 P1. I compared this to the gcode exported from cura 3.4.1 which just had M106 S255. I first tried to modify the gcode to have M106 S255 P0, AND THAT WORKED Later in my investigation I noticed in the "Machine Settings" under "Extruder 1", there is a new setting called "Cooling Fan Number", by default it was set to 1, hence the "P1" reference in the gcode.
Change the option under "Cooling Fan Number" from 1 to 0.
After that I saw cura exported the gcode as before M106 S255. I popped in sd card and it seemed to solve my issue. I am sure maybe in the Marlin configuration you could change the fan number as well but this seemed to fix my issue.
Hope this helps!
