With O27 I suppose 4 x 8 is fine, but that's a sub-standard radius and very limiting. I wouldn't even build a layout with O31 these days, unless you are totally committed to Postwar. Since O36 is the new O31, I would use that and *definitely* choose 5 x 9 if you have the space.
With either of these sizes you are talking about a free-standing "island" layout. 48" is too wide to reach across, so in either case you should leave a maintenance aisle between the long side and the wall. 36" is considered the limit of arm's reach, so in theory you could even build 6 x 10 or 6 x 12. As long as you have a maintenance aisle on the back side, you would still be able to reach the center.
5 x 9 used to actually be more common because it's the size of a ping-pong table. I think most people use 4 x 8 to save space, or because it's all they can get at Lowe's or Home Depot. My vote is 5 x 9 if you can get it, and if you have the space.
