Place all of them in, when your PC freezes, take out the 2 on the black RAM slots, check if your PC is working and does not freeze.
Then check the amount of RAM you have, go to computer then look at the bottom for the amount of memory you have, make sure it is 4Gb.
(If you already knew this sorry, just making sure)
Then replace the RAM with the other 2 sticks you have, so swap them.
Make sure there is 4Gb.
If at any stage you get less than 4Gb than a RAM stick is broken and will need replacing.
When you have done all of this (and they work), place 3 sticks into your PC, two in the white slots one in one of the black slots, check to see if you have 6Gb RAM, then swap the RAM in the black slot to the other slot, and check if you have 6Gb RAM.
If you do not or the PC freezes then it may be your motherboard or RAM slot(s).
If this all works repeat with the remaining RAM stick, and see if 6Gb is the amount of RAM you have.
Then place all of the sticks in, making sure you have 8Gb RAM.
Does your PC still freeze?
If it does then you will need to test with another manufacturer of RAM (if not possible take it to your local repair shop, and tell them they should be able to test it better and in more detail)
The other thing you could do would be to run memtest86 and see if you get any errors. This would tell you if you have a bad stick of ram or if you are going beyond your memory's speed (if you are overclocking at all).