Yes, you can use four RAM slots. However, whether or not it will be beneficial to do so depends on a few factors. First, your motherboard must support quad-channel memory in order for four RAM slots to be of any use. Second, even if your motherboard does support quad-channel memory, your CPU must also support it in order for the extra RAM slots to be utilized.
Finally, you should only use four RAM slots if you need the extra capacity or if you are looking to achieve higher speeds. If you only need 8GB of RAM, then there is no point in using four RAM slots since you will not be able to take advantage of the extra capacity. And if you are not looking to achieve higher speeds, then using four RAM slots will not be beneficial either since you will only be using two of the four available channels.
