CyanogenMod's maintainers are on an everlasting quest to conquer the software update war against pesky smartphone manufacturers who drop support way too soon for most of their devices. For instance, Samsung has been struggling to release Marshmallow for the two-year old Galaxy S5, but the CyanogenMod folks are ready to roll Android 6.0's software to an even older flagship: the three-year old Galaxy S4.
In this case, the CM 13 nightlies are available for the AT&T and Sprint versions of the Galaxy S4 (jflteatt and jfltespr respectively). The international S4 got it a couple of months ago, but other carriers and variants are still stuck on CM 11 and CM 12.1.
Remember though that since these are all nightlies, bugs and issues are most likely present. You'll also need to flash the Google Apps ZIP package if you want to have access to the Play Store and other Googly apps and goodies.