categoy page grid

hi,
first of all congratulations for this beautiful theme.

i’m using it for a portfolio website and i find the 4 rows grid very comfortable.
but i want to know if is it possibile to divide one of the squares of the grid in a grid composed by four little squares, and link them to different posts.
is it possibile to do with the css or i have to create child theme?

i’m good with css but not really good with php.

thank you