Many high end 4k tv's are true 120hz panel (for example Samsung's 8 and 9 series). Everything over 120hz is considered "effective". So Samsung 2015 high end tv are true 120hz (meaning they really refresh at 120hz) and are 240hz effective (meaning they will "achieve" 240hz through image processing like interpolation and such).
Every company has its own naming scheme. Samsung for example uses Clear Motion Rate and the number is always the double of what the tv true panel refresh rate is. So a 240 CMR is a true 120hz panel while a 120 CMR is a true 60hz. It's a marketing think and notice how they don't use the nominal "hz" next to their CMR number on their boxes.
Now, there's also the input refresh rate supported by the tv when used as in PC mode as a PC monitor. For example, the same high end Samsung TV is a true 120hz panel but won't accept anything over 60hz in terms of input (even at 1080p). So even if your panel can refresh at 120hz and you push 120fps, it will act as a 60hz PC monitor. It refreshes at 120hz only for tv viewing. Up to now, absolutely no tv can accept input higher than 60hz at 4k. At 1080p, the only tv's that do refresh higher than 60hz are Vizio (they do 120hz at 1080p). While Vizio is not as top notch as Samsung and Sony in image quality, it can be a very affordable alternative for gaming (mostly at 1080p) because you will get a true higher refresh rate.
The other thing to consider though is input lag. The fastest tv's on the market right now are around 20-23ms input lag and many will be in the 30ms in "game mode". It's totally playable but it just can't compare to gaming monitors like the ROG swift (1-4ms depending on TN or IPS) with their 120-144hz. It's a tv after all and to achieve such high quality image, the tv needs to add processing to the image. Game mode will disable most of the extra processing but not everything. Overall, it can act as a PC monitor but it's not its primary purpose
If you want panel information, the best site around is http://www.rtings.com
You can check by brand and get all the information summary needed + in depth reviews. As an exemple, here is Samsung's page:
http://www.rtings.com/tv/reviews/by-brand/samsungEdited by Merranza - 1/7/16 at 6:52pm