what is the diameter difference between these 2 say at 40#? Is the 8 carrier the same diameter as 30# braid (ie thinner than 4 carrier)?

I can't speak to the diameter, but I will note this: The 8 carrier is much softer and better to fish than the 4 carrier.

With the Toro produce, the 8 strand use the same fibers as the 16 strand hollowcore. These are better fibers than are used on the 4 strand. The 8 strand is thinner, and round by comparison to the 4 strand. Exact diameters I can not really provide, factory did not update our specs as fibers improved over the past few years. I can offer that we see greater capacity on spooled reels, if a reel held 300 yds of 4 strand 50#, then we will gain a good 50 yds with the 8 strand and hollowcore will gain a bit over that. Talking with some of our clients who make use of spinning gear, many prefer the 8 strand, feel it's casting properties are better but I really can't offer an opinion in that respect. The more strands, the smoother the line, nicer it is on the fingers, less abrasive. For a kelp cutter type rig, 4 strand can be preferred.

8 Strand and 16 Strand