If it's 4.25A @ 5V, then you're looking at around 10A (where'd you get 20A from?) for it nominal - inrush current could easily be 40-50A.
The diode is definitely a good idea to avoid reverse spikes blowing your psu up, I tend to use a rectifier diode usually (1N4001 or such).
BTW to handle 40-50A of surge you're going to need some badass caps - most have a ripple current rating of less than 10A.