Samsung Galaxy S4 already gets CyanogenMod 10.1 nightlies
For a time, it was unclear that CyanogenMod would even support the new Samsung Galaxy S4, but that's when Steve Kondik stepped in. Kondik founded CyanogenMod, and worked at Samsung for a while, leaving the company about one month ago. Kondik went back to CyanogenMod after leaving Samsung, and he has already finished the first nightly builds for some Galaxy S4 models.
Right now, the only variants of the S4 that is getting the nightlies of CyanogenMod 10.1 are the T-Mobile model (SGH-M919) and a Canadian variant (SGH-I337M), but Kondik says that as soon as the source code drops for other models, it should be pretty quick to put together nightlies for those as well. These first builds aren't perfect, but Kondik says it should be good enough to be your daily driver. And, the team is already getting to work on supporting hover gestures for the S4.
The first nightlies have been posted, although they would be a bit tough to find if you didn't know where to look, because they aren't listed as "Samsung Galaxy S4" or even by the model number. The T-Mobile variant is listed as "jfltetmo" and the Canadian model as "jfltecan".