The extruder axis E could be used as 4th axis if you have no extruder. For firmware V2 we plan up to 7 axis, but that is currently not that usable. Still in early development stage.
Only the options for X, Y and Z axes appear in the Mechanicals menu.
In the Tools menu there is the option to add a new extruder, but the extruder options are only Extruder 0 to Extruder 6, as shown in the attached image, which you already know very well.
E axis is only available by adding an extruder. Extrude r0 is the E0 socket on RAMPS so what you need. Since it is no extruder select a fake thermistor to get no errors and run M302 S1 to allow extrusion (E movement) while extruder is cold.
Not sure for what you need a 4th axis, but E axis gets some special treatment just because it is intended as extruder motor. So it can have different speed (flow multiplier) and has it's own jerk/start speed settings.
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For firmware V2 we plan up to 7 axis, but that is currently not that usable. Still in early development stage.
M302 S1
to allow extrusion (E movement) while extruder is cold.
Not sure for what you need a 4th axis, but E axis gets some special treatment just because it is intended as extruder motor. So it can have different speed (flow multiplier) and has it's own jerk/start speed settings.