One suggestion would be to replace that 4th 74hc595 with a tpic6b595. These chips work identically to a 74hc595, but their outputs can sink up to 150mA each, which would be more than enough and no transistors would be needed. (Note: the tpic chip's outputs can only sink current, they cannot source any current.) But unless you happen to have tpic6b595 on hand, you would need to order some, and in that case you should order max7219 or ht16k33.