I didn't use one, but generally you should base it on the current that will be consumed by the driver.

So, if the output of the driver is 4A at 50V, then that's 200W (multiply the two together). Then, divide by the voltage input of the driver, say 100V. This means 2A.

However, that does assume there's a 100% efficiency in the transfer, which there never is. I'd estimate between 75% and 90% efficiency, so worst case, it will be 2 x 100 / 75, or ~2.7A. Then choose the next value up from there (probably 3A).

This is also applied to the power supply itself, so if you had 3 drivers with 2.7A@100V (270W x 3), then that would be 810W. Giving 220V into the PSU, you'd need a 3.7A, but with 75% efficiency, it's 4.9A - or say 5A.

I'd probably be tempted to use a self resetting fuse - if anything goes wrong, just remove the power, and the fuse can be reused.

As to the type of holder - really up to you.

One thing though - if your motors are rated at 4A per phase, then wiring them in parallel will actually consume 8A, since in parallel, the current is doubled (you're putting two phases in parallel).