Border of each tile probably = lacking a proper tiling texture. If you are sure ( 100% ) that this texture works as tiling in the context of game assets, then maybe VAM has a problem to apply a tiling texture. But knowing I made tiling cubes that works perfectly in my first scenes years ago and did not had that problem, it's probably the original file being incorrect.
Now, a "ground texture" is not A texture, it's generally a complex shader composed of multiple textures and methods to blend them together. The general resolution you'll find nowadays is around 1024 per square meter. Cyberpunk, has almost no 4K textures for instance.
If you want a high fidelity ground, you will have to dive into Unity, build your own shader and ground and texture it properly to leverage both, the advantage of a nice shader and the small scale of 512/1024 texture and keep a proper texel resolution.
Using a single 4K texture will always feel bad if you stretch them a lot, or if you keep them at size and don't make a proper tiling.
Generally the first easy approach is to use a triplanar shader with a nice tiling texture. But it also depends what sort of size (for the enviro) you're aiming for.
But unless you're doing something wrong, a 4K texture should not feel blurry
