needed. How high is this ladder going to transport a person? How heavy is the person persons using the ladder?
Racking of a homemade ladder is one of the problems that could cause accidents.
I would consider using 2x4 side rails, all cut at angles on the bottom, then use 2x2 treads cut into the sides of the 2x4. They are actually knotched in and that way will not slip or move. You can glue them in place or predrill a pilot hole and screw them to the 2x4.
Without knowing more info, I would be leary of 1x4 sides.