I don't think the idea is to have the assembly weigh less, but for the overhead of the truck and trailer to be less. 4klbs less trailer means 4klbs more cargo.
The single tire trucks and trailers are on the road today, at least here in the northeast US. I haven't heard of any accidents caused by the tires, but advances in tire technology may make them less apt to blow out than semi tires years ago.
Some carriers go all-out and also install fairings under the trailer and around the trailer doors. Particularly the long under-trailer pieces could probably benefit from whatever manufacturing advances allowed them to produce the trailer side panels.