But if 2 or more LEDs are turned on the current would add, giving a greater voltage drop on the common resistor, so reducing the forward voltage, so reducing the current. The current would be shared not quite equally, but shared and limited.
Maybe an option would be a PAL device programmed as a 3 to 8 decoder with polarity switch (for 4 signal ins) and having 2 commons, which would make a 6 pin 4 digit 7 segment driver possible. This would free up 4 more pins.