Is it worth it to buy larger stepper drivers?

I need 2 stepper drivers to drive two 4.6A stepper motors.

The DM556 driver is optimal, but only provides 5.6A peak (about 4A RMS)

Is 4A enough for a 4.6A motor?

I currently have one DM556 driving another 4.6A stepper motor (I have 3 identical motors for a XYZ CNC router), and it seems OK.

Is there a major downside to reduced current?

I would just set all of your motors to 4.6 amps and see how it goes. A 7 amp power supply would probably be enough (should be about 8.3 amps by my 40% rule of thumb)

I don't have a driver capable of 4.6A.

I have one DM556 driver that is capable of 4A RMS maximum.

That is all.

RMS is not "true" amps for some static amps for holding. But if yiu can test it now and its holds yout torque and it is not too hot - all is ok.

My mains voltage is 110V

I already have a toroidal ransformer and rectifier bridge capable of supplying quite a lot of current @ about 40VDC and about 40VAC

I was thinking about buying this one: www.newegg.ca/p/1W7-00T2-00FM3?Item=9SIBJRKJRJ5551

I would just run with what you have and save your money for now. You just might be surprised!
Just because a stepper motor is rated to 4.6 amps does not mean it will use all of that in your application.

For example, yesterday I did a motor design for gantry. The motors are rated to 2.0 amps but the calculations say it will use 1.3 amp peak and 1.0 amps RMS. So if I set the  drive to use 1.5 amps instead of 2.0 amps, it would make no difference to performance. 
The calcs say I need at least 48 volts to meet my design parameters

So here is a humble stepper able to run at 60 m/min rapids and 5 m/s/s accelleration on a basic 48 volt power supply. Initially, I thought I might need servos but I have a bunch of these Sanyo Denkis on hand.