4” from Shars is likely as good as you need for your machine. I have one and am happy with it.
John
The Table is 6” wide 16” long with 7/16 T slots.This is just me speaking. That vise is more of a drill press vise. I like my mill vises to be low profile and very stable. I am not familiar with the table size of your Smithy. I would opt for a good quality 4” vise. Maybe even a 3” if you work with smaller parts. You can always use a sine bar to set angles when needed. I would much prefer solid and low profile to maximize working area and clamping force.
Cutting oil is my blood.