Pins 3 and 5 (GND) are usually shortened by a button, according to the linked manual, so the 4k7 resistor is not required where you draw it.
But I'd add a resistor between attiny and transistor base.
I guess your 4k7 might work. Depends on the pullup resistor, which is obviously built into the HackHD.
I bet dhenry would say you can even do without transistor and resistor:
simply connect the attiny output to 3 attiny GND to 5 and set output to low for active.
I understand there's no warranty on neither a hackHD nor an attiny45, so just give it a try
If you're timid, you may measure the voltage of Pin3 and the current flowing between Pin3 and Pin5 through a DMM first.
Should be about Vcc and very few mA.