CodeFights: Game 2048
The game2048 function is short, which is good. It could be more clear to the casual reader.
The 2nd grid assignment uses zip(*grid), which is fine, but it would benefit by being broken out as a named helper function, which the third grid assignment could also use. Helper names often do a better documentation job than comments do.

The merge function uses the identifier transposed, where perhaps row would have sufficed, without confusingly calling to mind the 90-degree aspect of matrix transpose.

The magic number 4 is distracting - please assign a board size constant and use that.

The code lacks comments, which is not necessarily a problem, especially since it tries hard to use meaningful identifiers. Its clarity would benefit from unit tests (or docstring tests) showing before / after effects on a grid or row. The "output should be" narrative is helpful; I'm suggesting writing it as a TestCase. In the course of doing that, you may find it useful for coverage or didactic purposes to break out the occasional helper function.