Any image you take with the 18-135 will be fine on 4k. Even my K20D, K-5, and even my 6MP *ist and K100D images look fine on 4k. 4k is 8MP but that doesn't mean it can't upscale nicely.
Do you know how to read a lens chart?
My second piece of evidence....
The 18-135 will be fine for what you want, even overkill. You are going to be reducing the image from 6000 pixels wide, to 3850 wide. Most documented weaknesses in the lens will be hidden by reducing the size of the image. An 18-135 reduced to 4k is as good as any ones out there. Because of the field curvature at 18mm, I wouldn't use it for panos, but by 24 mm, that's cleaned up.
I look at over 600 images every time I sit down with a beer. I have never once said to myself "that image has soft edges it must be the 18-135."
K-3 and 18-135 at 24mm
Soft edges my derriere... pffft.
I bought mine when it first came out roughly 7 years ago, and I've never seen a practical reason to replace it.