This is a bit of a sticky situation isn’t it!
It is important that you have all 4 as that will apply even pressure to the die of the chip as in even pressure can crack is in half and kill the chip instantly
I would buy a new heatsink to be on the safe side but if that isn’t possible I would use 2 pins that are diagonally on ether corners of the heatsink to apply even pressure (refer to image below)
You can use the ones circled if you would like
As for the idea of using a screw: I wouldn’t do that as it would be hard to control the pressure as the heatsink east designed for it
Hopefully this helps
Any questions please ask