I like the idea of casters so the layout can be moved. Things seem to grow under immovable objects.
My layout will be in the basement on a smooth, relatively flat concrete floor. I had no problems moving my tools on mobile bases but each weighed hundreds of pounds and they were on 3 inch HD casters.
For reference purposes what's a 4x8 HO layout weigh? 2x4 with 3/8 OSB or 1/2 inch ply top.Just a rough idea as I'll up the recommendation to be on the safe side. I subscribe to the old belt and suspenders theory, you never can be too safe.