How is it my tv it doesn't make any sense. When I watch tv never did the screen ever dim or get brighter at times. And never when playing on my PS3 or 360 did the screen ever dim mid gameplay.
My thoughts are that the light sensor on the PS4 changes according to the light in the room.
If you actually want a solution you need to go through the basic troubleshooting steps first—check your TV's settings. With most modern HDTVs you have a different settings template for each input. For example, on my TV I use dynamic contrast and the eco sensor on HDMI 1, because it's just my cable box. But on HDMI 2 and 3 I have those settings disabled for my gaming consoles.
You're most likely using a new input for your PS4, which means it hasn't been configured yet. Default settings almost always use the eco sensor.