Physically, yes, as long as it matches the type required by the mobo.
As to whether it will work, it's as good as a coin toss. It may work fine, or it may not work at all. Can you provide a bit more info about what mobo you have?
i purchased an 8gb stick and i just realized that my mobo only supports up to 4gb, will i still be able to install the 8gb stick?
Well, firstly mention the motherboard model number and the CPU model.
If you are sure that the max is 4GB per slot and that single 8GB is not compatible then I suggest don't go with 8GB stick. Instead get the 4GB module. The chances for single 8GB memory to work is little to none.
I have an HP Pavilion e9105z with 6gb of DDR2 ram. i just purchased a Patriot Viper Xtreme 8GB DDR3 1600MHz ram stick to put into my computer but i just realized that the mobo that comes with this pc only supports up to 4gb per slot as it says on the website