How will that help? Decade counter only has 1 output high at a time.
I think you'd be better off with TPIC6B595, one per digit, and drop all the transistors.
Simpler code too:
// did a digit change that requires a display update?
if (updateFlag == 1){
updateFlag = 0;
digitalWrite (csPin, LOW);
SPI.transfer (digit0); // lowest
SPI.transfer (digit1);
SPI.transfer (digit2);
SPI.transfer (digit3); // highest
digitalWrite (csPin ,HIGH);
}