Remove your window Skirtboard and replace with a new one to meet your demands of 3" as an option.. I would not use 1 single row of 4" .

-good luck

I'd just do the subways and have a very skinny cut row under the sill. It will be almost invisible, but consistent and look right where the row leaves the window and continues full sized. The row of 4x4s wouldn't be good. And yep, changing the window trim is also an option (or adding a bit of additional molding under the sill).