D.B.
I just want to add that you can do some other things if the CDs don't pan out, or just to break up the monotony. Don't be afraid to use sing-along CDs with American classics - as a music teacher, I can't tell you how much young children enjoy things like "Take me out to the Ball Game" and "Home on the Range" and things from 50 years ago. You can probably borrow a bunch of CDs from the library at no cost - having the parents participate is a huge benefit to kids! Your little one won't' get all of it, but will enjoy everyone singing. Then of course there is another generation of songs like "Baby Beluga" and "Wheels on the Bus" - anything with hand motions is great, and your 1 year old can follow along.
Also play "I Spy" looking at things out the window - could be animals or barns or bridges or whatever, or colors (green fields, yellow signs, black cows) -- great for the 4 year old, and the 1 year old can observe. Or do it when the little one is napping and the older one is driving you nuts.
Depending on your 4 year old's abilities & interests, you could try a travel bingo set - some of them are standard bingo board - 5 x 5 - with pictures of common highway sights. Some have sliding "windows" you use to cover sights you've seen, others are magnetic, and still others are just sheets of pictures with a check-off box (if your child can manipulate a pencil or crayon).
Consider an activity table that fits across the car seat, so a child can color or read, or have a table for a bingo board.
Anything your 4 year old can do that involves a whole ton of singing or call/response or hand motions is a GREAT experience for the little one. That visual exercise is a great option to the purely aural experience of a book on tape. I'm not saying don't do the book on tape - it's one option among many - but it's very hard for a 1 year old to follow that for a long period of time. And if the 4 year old is a visual learner, you need something other than audio.
So, mix it up and have way more stuff than you think you need!! The public library is a tremendous resource that lets you borrow at no cost. And check with the children's librarian for more suggestions.