is there still a benefit in 4K textures even if the output res is (much) lower than 4K? If so, is there a resolution breaking point where it becomes useless to have 4K textures?
Yes and maybe.
Screen resolution is a 2D grid of colour values referred to as pixels. Texture resolution is a 2D grid of colour values referred to as texels. If you have a 1080p display and a 16:9 aspect texture of dimensions 1920x1080, and you draw that texture to exactly fit the screen, you will have a pixel perfect texture. If you have a 16:9 aspect texture at 4x the resolution, 3840x2160, and display it on that display, you'll either view every other pixel or average a group of four of them. If your texture is only 192x108, you'll have to interpolate pixel values from between texel values for every ten pixels.
That's when the texture is perfectly aligned with the screen, but textures rarely are. If you enlarge the texture to fill more of the screen, you reduce the number of visible texels being used to create the pixel data. Thus, walking towards a wall or shooting the wall with an Engorgeo Ray, the original 1920x1080 texture no longer provides a texel colour value for every pixel and instead you start to interpolate values and create blur. The '4K texture' will be able to provide per-pixel data up to viewing 1/4 of the texture, beyond which it'll start to blur. On higher resolution displays, the perfect texel
ixel ratio is obviously requiring higher resolution textures. A super-massive texture of 16k x 16k could fill both a 1080p and 4K display with pixel-perfect data even on close zooms/large scales. Let's say zooming in an object through a sniper lens or binoculars. Uncharted 4 has some pretty horrible low-res assets when zoomed in. If they were many times the size, you could zoom in and still see detail instead of blur (this goes for geometry detail too, of course)
However, if your game is such that you never need more texel data than your display requires, it'll go to waste. Imagine a top-down game where no textures are stretched larger than the screen - extra texture data will never be used.
It's also worth noting that '4K texture packs' aren't necessarily perfect for a 4K display, any more than the 1080p assets are perfect for the 1080p display. You are still likely to be interpolating texel values to fill the screen because the per-pixel data is too much to fit in RAM, unless you used tiled assets/megatexturing, which no-one is doing, despite it being the future, which I guess it wasn't, but I don't know why.