I see no reason why it can't be on castors.

I don't know. Rolling around 400lbs could be difficult?

My sister had her cabinet maker build a huge honkin' solid maple island on casters. You can barely see the casters. The whole thing looks built in. Until it's not. Only downside is no electrical, so will that matter to you?

Where I live, an outlet is required in an island. Theoretically. I guess if it's on casters you can just roll it away until the building inspector drives off. ;-)

I think building codes require outlets on fixed islands but not moveable ones, regardless of size.

MizLizzie - thanks for sharing that your sister has done something similar on a larger scale and that it looked fixed but was not (that is thelook i want).

GauchoGordo 1993, rom718 ,Errant:
I'm not needing electric in the island and it was also my understanding that electric isn't required on a moveable island.

I'm really not planning on moving this overly much, really only when I want access to the lighting fixture above the island on a tall sloped ceiling, so just want the ability to move it out of the way of the ladder.

I currently have an island on wheels and it is really functional and the exact size, but it doesn't look very nice and I was hoping to coordinate the quartz countertop which is what is driving me for this now. My contractor is going to work with me on the design of the island, so maybe some give and take on the elements I want.
Thanks for the comment.

It may depend on jurisdiction, but when we remodeled our kitchen in our previous house about six years ago we did not have to put an outlet in our island because it was not fixed. Ours was not on castors, but it was not built in--it just stood on four onion-style feet like a piece of furniture.