UP FOR DEBATE : what type of texture is used nowadays in gaming industry 1K , 2K or 4K

It depends on the type of the model and on the type of the game as well. There's no need to use 4k on all models when you know they will just fill the background or will be only small props that you won't see in full size on screen. Only the main models would benefit from a 4k textures. It's also different for a shooter game and for example a strategy game. You're supposed to see big models in a shooter but you won't probably see one model fill the whole screen in a strategy game where you have tens or thousands models so you shouldn't waste the power on big textures which won't be seen in much detail.
Also when you make a mobile game, then 1-2k textures would be enough for most of the models (I still see many games still use 512). Only when you do a console or PC game then you should use the maximum resolution that the engine allows you.

Richard is correct. Also, I'd like to mention that resolution can be considered 1-2-4K if used the way it comes and fills UV space in its initial resolution. UV mapping can cut quality off easily. Eg. 4 walls unwrapped on one 4K texture atlas next to each other to fit in, (and you are using a 4K brick wall power-of-two image) is not 4K quality because each wall will use only a portion of the 4K texture image - so you will have ~1K resolution wall textures due to having to shrink them down to fit (that is of course if you want to apply decals and such and want to make each wall look different). Else you can have them overlap, retain 4K quality and have all 4 walls look exactly the same.
To pick an example as to what Richard mentioned: iPhone 5 and up handles 4K textures but its predecessors don't. If you look at my PBR prop pack in here, I used a 4K texture for 8 trash cans - so I roughly got 512x512 resolution for each trash can, and it's enough in that case.