We're not referring to the bed being level to the ground. Think about this is terms of geometry. The printer's bed is level when its z plane is orthogonal to the printer's x and y planes.

As the bed size scales larger, small errors in the bed level scale exponentially. For this reason larger beds are harder to level and require the level to be more accurate. Printers such as the Neptune 4 Plus and Neptune 4 Max present additional challenges for bed leveling.

Also known as “tramming”.

Elegoo's Quick Start instructions and the workflow on the ancillary side Screen controller use what's known as the “Paper Method” to attempt to level the bed by moving the nozzle to each bed screw position where the bed height can be adjusted and testing the tension of a piece of paper. There's several reasons this is problematic, such as

Note: Feeler gauges shouldn't be used either. They will damage both the plate and the nozzle. While a specific thickness, they too are subjective and at best your accuracy using them not as good as what we can measure with the z probe.

The Neptune 4 Series printers have an accurate z probe that can measure the height of the plate at any position with a sub-micron resolution of under 0.00250mm

Recall that in geometry a plane is defined by at least 3 points in space. If we use the probe to measure the bed height at three (or more points) and the Z height agrees, the plane would be level. If we take those measurements at positions on the bed where we can adjust it's height, like the bed screw positions, we can adjust the screws and measure them and when we get each of the bed screw positions measuring the same Z value, the plane would be level.

Klipper has a built in feature to automate probing and measuring the bed height at the current location, PROBE, and MANUAL_PROBE.

If we have some basic knowledge of the screw threads we'd understand by how much a rotation would raise the bed. Used with probing the bed height and knowing the target height to level, Klipper can figure out how many “minutes” of rotation the screw needs turned and in which direction (clockwise/counter-clockwise). These “minutes” are like the minutes on a clock, 15 minutes would be a quarter of a full screw rotation,

Klipper also has a built in feature, SCREWS_TILT_CALCULATE, to automate this testing and to measure both the how level the bed is and how many turns each bed screw would require to achieve being level.

It requires knowing the exact position the probe needs to be at to be over each bed screw. (Note we want the probe over the spot we're measuring, the bed screw, not the nozzle. The probe and nozzle are offset to each other in the print head, with the probe in the rear left corner of the head.

See https://www.klipper3d.org/Manual_Level.html#adjusting-bed-leveling-screws-using-the-bed-probe

The first defined bed screw is the one that is used as the reference position. All the other bed screws will be adjusted with the exception of the first one defined.

The Neptune 4 Plus and Neptune 4 Max have two middle bed mounts that are fixed. As neither of these can be adjusted, we shouldn't define them as one of the bed screw that will be adjusted. Instead, use the position exactly in between the two mounts. For example, if you had a N4 Plus and the middle-rear bed mount was measured to be at 189.25,204.55 and the middle-front bed mount was 189.25,86.55, use 189.25,118.00 which is the position in between.

screw1: 189.25,118.0
screw1_name: btwn middle bed mounts

While it's best to measure them yourself, for your bed, which could be different, the following files describe bed screw configurations for each model Neptune4 and could be used for quick start and to confirm your's aren't far out of the ballpark.

https://github.com/OpenNeptune3D/OpenNept4une/blob/main/printer-confs/n4/section_screw_tilt.cfg https://github.com/OpenNeptune3D/OpenNept4une/blob/main/printer-confs/n4max/section_screw_tilt.cfg https://github.com/OpenNeptune3D/OpenNept4une/blob/main/printer-confs/n4plus/section_screw_tilt.cfg https://github.com/OpenNeptune3D/OpenNept4une/blob/main/printer-confs/n4pro/section_screw_tilt.cfg