I believe the grid lines are one pixel wide. What size is that 4K monitor? If the PPI (pixels/inch) is really high, the grid line might appear faint.
If you have the in-game anti-aliasing enabled it will make it worse.
Higher density (pixels/inch) monitors do not necessarily translate to clearer visuals, for some things. Like the single dot when a plane comes just into view range. The biggest gains with a high pixel density monitor is going to be the reduction in aliasing.