AMD is collaborating with the "industry's biggest scaler vendors" to produce dynamic refresh hardware for PC monitors. According to the official AMD press release, MStar, Novatek, and Realtek have all committed to producing chips that exploit the Adaptive-Sync capability built into the DisplayPort spec. FreeSync is based on Adaptive-Sync, and each scaler vendor is working on a "range" of options that should appear before the end of the year. Those scalers are destined for monitors due in the first quarter of 2015.
Although FreeSync is based on a royalty-free standard, there are some strings attached. Only certain Radeons will work with the technology. Also, the initial batch of scalers appears to have some bandwidth limitations. The wording in the press release is a little awkward, so I'll just cut and paste:
Finally, the initial Project FreeSync-compatible scalers from MStar, Novatek and Realtek will empower monitor vendors with robust DisplayPort™ receivers accommodating of FHD and QHD panels up to 144Hz, or UHD panels up to 60Hz.
Sounds like 4K FreeSync displays will have a maximum refresh rate of 60Hz to start. Monitors with lower 2560x1440 and 1920x1080 resolutions should scale up to 144Hz, the same peak rate as Asus' QHD G-Sync monitor. Acer has already announced a 4K G-Sync display, but we don't know whether it will be capped at 60Hz.
Any monitor based on the Displayport 1.2a spec will probably be incapable of cranking out 4K frames at substantially higher refresh rates—there simply isn't enough bandwidth in the interface. The latest DP 1.3 update boosts the interface speed enough to support 4K displays at up to 120Hz, though. Gamers who want to combine ultra-high resolutions with ultra-high refresh rates will probably have to wait for displays based on that newer standard. They'll also need an awful lot of GPU horsepower to keep up.