A 2x4 insert pattern means your board has two rows of binding holes spaced 4cm apart along the length, but only 2cm apart across the width. Compare that to the older 4x4 pattern, where holes are spaced 4cm in both directions. The tighter 2cm cross-board spacing is the key difference.
The big advantage is finer stance adjustability. With 2x4, you can tweak your stance width in smaller 2cm increments instead of jumping 4cm at a time. If you've ever felt like your stance was just slightly too wide or too narrow on a 4x4 board, 2x4 gives you more options to dial it in perfectly.
You'll find 2x4 on many modern boards from various brands, including Burton's non-Channel models. The good news: most standard bindings mount right up with no adapters needed. It's increasingly common and worth seeking out if you like experimenting with your stance.