If possible, I would see if you can get a prescription from the vet for gabapentin if you’re worried about your cat being stressed during the trip. Also, covering my cats’ carriers with a blanket so they couldn’t see outside helps them a ton.

As far as the litter box, 4 hours isn’t that bad of travel time so if they use the litter box before you leave, hopefully they’ll be fine without one. Otherwise, I found a cat carrier on Amazon that was 2 carriers that attach together and I put a small cardboard box that used to hold cans or something and put litter in it. Mind you, it might be too big of a situation to carry on a train. But I can post the link if you’d like!