Transporting new cat with resident's cat blanket?

You should reduce the stress to a minimum, and that means reducing the sensual input. Being caught in a place with the intense scent of another cat will probably stress the new cat because she "invaded" the territory of another cat (against her will) and cannot escape. Put a clean towel into the pet carrier that is easy to wash if your new cat has to relieve herself.

She should not be able to look out the window, because all those images are too much input for her. If she's nervously looking all around, consider covering all but one side of the carrier with a light, breathable fabric like a thin bedcloth. Make sure the carrier doesn't get overheated or stuffy, but also don't let air blow directly into carrier (that might cause irritation of the eyes and nose).

In my experience, every attempt at reassuring or relaxing the cat by talking to her or even touching her will result in the exact opposite, because she doesn't trust you yet. It'll be probably best to ignore her during the ride.

Once you arrive at home, put the carrier into a room with water, food and a clean litter box and open it. Make sure the cat cannot dash out of this room. Then leave the room and let the cat calm down for several hours.

I would give her several days in this room to get used to her new surroundings. During that time you should of course start socializing with her. Since you already carry the smell of your other cat, you'll automatically start the process of introducing both cats.

As to how to best introduce 2 cats, please refer to these related questions:

• Recommendations for introducing adult cats to newly adopted adult cats
• Recommendations for introducing kittens to adult cats
• Using the room door as a means of introducing cats to each other slowly

I might be a little too late to answer this, but 4h is a very long ride for a cat.

Contact to a vet about giving him a light sedative. Cats sometimes receive human sedatives, of the top of my head Xanax, but never dose it yourself.

In spite of the age difference, 1 year old cat would be more stressed compared to the 4 month old kitten because getting used to a new place is much more stressful than getting used to a new playmate. So, use a clean towel as Elmy suggested and give the towel to your 4 month old kitten upon arrival.

Since the cat doesn't know you, there is no guarantee that he will be comforted by you. Still, experiment with soothing tones. If this works, you can keep doing it. If not, ignoring is a good option.

I transported cats a few times and with a light sedative, they nod off in their container and had a peaceful trip. But all those cats either knew me or someone else in the car, so there is no guarantee your trips will be as peaceful.