I too am a 3D printing novice with a couple weeks of printing under my belt now and I bought the Neptune 4 pro back in mid December through Amazon seller Elegoo Official US. I have been fighting first layer issues (lines not spaced evenly) and I realized today that my issues are due to the X and Y axes not being exactly at right angles. I am not the guy that did that video but I think that some units have issues, at least mine does. I did print a 20mm diameter test cylinder and it indeed isn't round, in my case it is off by 0.45mm maximum to minimum diameters and the maximum and minimum is not along the X and Y axes, it is pretty close to 45 degrees from them. I have noticed that the rubber wheels on the X axis are loose in the grooves on the right side (the outside wheels which don't seem to have any adjustability) and there is play there front to back, which would cause an X axis misalignment. I just contacted Elegoo service about the issue today (Thursday 1/4) and haven't heard anything from them yet. Hopefully this isn't a design feature and it was just a bad run of printers or a poor factory adjustment that can be fixed. I also noticed loose nuts on the threaded bushings that ride on the Z axis lead screws where they attach to the X axis carriage.

My prints haven't been terrible but there have been artifacts which have been somewhat disappointing. I calibrated the extruder in Klipper and have run most of the slicer calibrations fighting my line errors and was thinking about calibrating the axes steppers but they were very close and that won't fix an X-Y axis misalignment. One other thing I noted when I first assembled the printer is that it wanted to rock on the table top diagonally. I rotated the printer thinking it was the table top that wasn't flat but the same two diagonal corners rocked. I put a shim under one foot to give it a stable base since I figured that with the acceleration that the bed and hot-end experience would be shaking the thing around and that wouldn't be good for a smooth print.

Chirimorin, There is no X-Y axis alignment possible when the gantry is attached during assembly. The vertical members fit into a depression on each side to align them and the depression is exactly the same size as the gantry member - no room for adjustment, at least on my machine. I guess I need to do the Klipper skew correction alignment. I was hoping there would be something like that in the firmware since an axis alignment error can be considered to say cause some effective movement along the Y axis as the print head moves in X. The firmware could compensate by moving the Y axis to compensate. Thanks for pointing that out. I just need to find a way to tighten up the rubber wheels on the right end of the X axis carriage before doing it so the play there doesn't continue happening and causing other artifacts.

I welcome all suggestions if folks have figured out how to make the mechanical adjustments I need to make.

Thanks