It wasn't even an M4 but an M3. But as I said, you are quite certainly missing something. Unless the US is about 1/3 cheaper than Europe, which I doubt. I'm well aware of leasing rates as my employer is one of the biggest luxury car leaser, biggest Porsche customer worldwide etc, and configuring cars is a companywide favorite for slow days. Although I drove a CLA for about 2 months, I don't know too much about Mercedes, nobody really likes them here. Way more expensive than BMW or Audi and generally worse cars. However, two recommendations for you:
1. If German, go for BMW (if you can handle the image). Best engines, cheapest lease due to highest resale values. Even their 4wd is par with Audi by now.
2. The surcharge for AMG/M/S models is completely out of whack. Get the biggest/baddest non-house-tuner-model and you have >90% of the car for something like 60-70% of the price. Example: Last year rate for S3 (36 months, 15k km) equal to A4 with 3l V6, 4wd, S-Drive, etc pretty much anything you can add. S3 has better sound, apart from that the A4 is better car in every way.