“On its own, it makes little difference so you need some holes in the official 4 case which is a very close fit. I suggest (take the pi out first) a 20mm hole over the heatsink and about 6 no 8mm dia holes around the sides, the GPIO side is easiest but one next to the USB2 ports wouldn't hurt and if it still gets hot add a fan but I haven't needed this so far.”
“I bought this as a stop gap as there were no PI4 specific heatsinks available at the time. It's rather too small but it does help. Dead easy to fit just peel and stick.
I have since fitted a somewhat larger heatsink. Even that's not really big enough (it;s about 20x20x20mm)
With the PI4 a little more care in cooling is going to be needed.”