Brad, if you read more, most people will suggest supported rails as a superior alternative to round shaft -- even with large diameter shafting, 48" is a long span for unsupported rails. But enough unsolicited advice .
For a machine this size, a Nema 23 G540 based solution makes a lot of sense. And if you're using stepper motors, you will want to go direct drive to take advantage of the lower end torque stepper motors provide -- a belt is not really necessary unless it helps with your mechanical mounting for some reason. We have sold tons of our 3 and 4 axis kits to people making machines around this size.