4" is fine but it'll be a little flexy. The lack of the wood being captive would concern me.
I used to think a steel trailer deck would be sweet...
Until I had one.
Everything slides on it.
You can't screw wood blocking to it if you need to.
For open trailers I have two. A 5x10 single axle no brakes with a filled (14 gauge steel) crash rail around it that is 12" tall. Wood floor, slide in ramps 60" ramps. Spare tire hangs underneath. The other is an 18+2 wood deck, two spare tires underneath, pinned fenders, and slide in 80" ramps.