Does this mean I could attach an LED to each Y direct to gnd and expect 4mA without damage? Does the loading require a resistor in series?
74HC00 datasheet: http://www.ti.com/lit/ds/symlink/sn74hc00.pdf
The short answer is no. It just means the output voltage will be at least 3.7V, when the output current is 4mA, indicating a maximum resistance of R = (4.5 - 3.7)/0.004 = 200Ohm. The typical figure shows 4.3V, when the output current is 4mA, which gives an output resistance of R = (4.5 - 4.3)/0.004 = 50Ohm. Again that's a typical figure, it could be lower.
Note, that this is with a supply voltage of 4.5V. If one runs through the above calculations again, they'll get higher resistances at lower supply voltages, and lower resistances as the supply voltage is increased.