If there's an all-white column, either one other column has two whites and we're done, or the other four possibilities for the other 6 columns must have a matching pair.
If there's an all-white column, either one other column has two whites and we're done, or the other four possibilities for the other 6 columns must have a matching pair.
So n≤8 is a bound in the 3xn and hence the 4xn case...
So n≤8 is a bound in the 3xn and hence the 4xn case...
(Or cropped screenshot, acknowledging that things like your battery percentage could potentially be extracted by someone who cared.)
(Or cropped screenshot, acknowledging that things like your battery percentage could potentially be extracted by someone who cared.)