I've never actually weighed my guys, but Frankie is pretty hefty.
I thought the egg crate would hold up (supported on three sides by the frame), but after a couple of weeks, I noticed she had cracked it down the middle

It's easy enough to fix - I just stuck a scrap piece of plexiglass under the egg crate. The ramp is still just egg crate, and although it flexes when she climbs up, it hasn't broken yet.
I started out with silicone on the ramp for Frankie, and hot glue for Ralph, but I've supplemented it with zip ties, since one end was coming undone.
For width, if 4" looks good now, then it should be fine for a while. As long as he's got room on either side for his legs when he's climbing up, it'll be good.