In a surprising announcement by the CyanogenMod dev team, they have announced that they won't be developing for the Samsung GALAXY S 4, or at least there's no plans currently to support the new device. Team Hacksung member XpLoDWilD says that "nobody at team hacksung...plans to buy it, neither develop for it." Team Hacksung is the group behind the official CM ports.
The dev team mostly blames the different variants of the new device, one being the Qualcomm Snapdragon 600 version in the US, and the international version which will rock out an 8-core Exynos 5 chip. They also note that many of the bugs present on the Galaxy S III will most likely be there on the GALAXY S 4 as well, most notably issues with camera support.
UPDATE: Steve “Cyanogen” Kondik himself has debunked this claim himself.
There's also the issue with Samsung locking down their platform tightly. Samsung wants to protect their IP, which obviously makes for very developer-unfriendly devices. Of course, the company works around carriers and such, but they haven't been too friendly toward independent developers as of late.
Of course, there will mostly likely be other ROMs available for the GALAXY S 4, and while other developers are following in the footsteps of the CyanogenMod dev team's decision to pass on the new device, there will still be some developers who want a stock version bad enough that they'll be willing to jump through the necessary hoops to make it happen.