Try using the "PULSEOUT" or the "FREQOUT" command for flashing.

Not quite sure how big your matrix is but standard connect 4 grid is 42 (6x7).

Thinking more in depth to your project you could take it a step further and make it a hell of a lot easier on yourself by implementing a MAX6952 Matrix controller.

but I like your project hope to see working video of it when complete.