Are you using display scaling within windows? You might want to set Unreal to not use screen scaling? (Right click on UE launcher, Properties, Compatibility, Disable screen scaling).
Well at 4K resolution a 1024X texture is now the new 512X texture so assuming your material is textured base you will need to up the resolution. This is a typical raster problem using textures and the current trend for material is towards vector, PBR or procedural materials.
On the other hand
From time to time my materials get super blurry but opening the material in the editor solves the problem so I never took to time to figure out why.