hello, i have my 4th axis aligned with Y that makes me problems to make g-code...so i want to swich  my X axis to become y and Y to become x axis so  my 4th axis would become aligned with x-axis... my 4th axis  is heavy so i dont want to  move it to current  x axis but i rather fix it  outside machine  and let spindle move in current y direction.. so i wonder,can i do this in mach3? where i can switch those 2 axis? beginer here...thank you

this is my current situation if this helps..

Not sure if I am understanding exactly what you want but if its just that you want the current X axis to move when you ask the Y to and vice versa then change the step and dir pin numbers around.
At the moment you have

X Step 2  X Dir 6
Y Step 3  Y Dir 7

Change them to

X Step 3  X Dir 7
Y Step 2 Y Dir 6

If thats not what you are meaning try again with your explanation.
Hood