Because of the delay in posting, any number of others may have already answered this, but we're at the mercy of the delay so have a great chance of being redundant in our replies... sorry.
The 4x4 and the grid are technically two separate concepts.
It is not the grid that is 4x4, it is a the logic problem that is 4x4 (4 groups of 4 items).
The particular grid shown is the grid necessary to represent all the combinations for a 4x4 logic puzzle.
There would actually be 9 (4x4 grids) to show all the combinations, but the identical intersections, i.e.where the column name and the row name are the same don't need to be shown, so those two (4x4 grids) are removed from the full grid, and the last corner (4x4) is redundant. It would show where c=b, but that relationship is already covered by the center 4x4 grid (where b=c).
So, removing those three (4x4 grids), leaves you with the six (4x4 grids) that represent all the combinations for a 4x4 logic problem.
The actual solution, only needs the top three (4x4 grids) to show the full result, the lower three just serve to keep track of relationships until you can transfer that relationship into the upper three (4x4 grids).
Even if the above didn't clear things up, probably reading some of the tutorials or watching the youtube videos on how to use the "grid" should make things clearer..
one of several youtube videos