4-variable K-maps have 16 squares which arearranged in 4 columns and 4 rows.
Columns and rows are labeled with 2 variables.
The rows are arranged so that C or D changes state for a single vertical movement but not both.
A single vertical movement between the top and bottom rows results in only 1 variable changing in the associated minterms. The K-map could be drawn on a torus (or doughnut) with the top row and bottom rowsadjacent to each other and the left and right columns adjacent.