What is the difference between four 1X dynos and two 2X dynos in heroku?

More dynos give you more concurrency. Based on a single threaded web server such as thin, if you have 4 x 1x dynos, you can serve four requests at the same time.

With 2 x 2x dynos you can only serve two requests.

2x dynos have more memory (1024MB) and more CPU available. This is useful if your application takes up a lot of memory. Most apps should not need 2x dynos.

Heroku have recently added PX dynos as well, which have significantly more power available.

You can read about the different dynos Heroku offers on their website.

The actual data changed since the accepted answer was posted.

The current difference is the memory (512MB for 1x and 1024MB for 2x), as well as the CPU share that is doubled for 2x.

More details can be found on the following Heroku dev center page: https://devcenter.heroku.com/articles/dyno-types