I recall seeing carts in the underground tunnels between 6/5/4 but it has been quite some time. But you'd still need to get down to the tunnel, hope for carts (if they still exist), and then go back upstairs at 4 and walk to TBIT. I haven't been down there in some time, but I don't recall if there were moving walkways. Given that your alternative for not walking would be to either (1) order a wheelchair, or (2) go landside and take a bus around the loop - do not do this - do not do this - do not do this, you are best walking, hope for a cart, and sit down or rest as needed, time permitting.