RAM - 8GB with 4GB or just 4gb+4gb (If you know stuff, come here)

Although you could use different capacity RAM sticks without much of a problem, you want them to have the same speed/CAS. Because of that I'd recommend going 4+4.

don't get 2 different types of ram
better to have two with the same amount. get 16 (8+8) if you really want to, but is there a reason?

It's hit or miss whether there will be issues with different RAM. You could look up specifically wit your MOBO, but usually the only way to know is to try it. I've never even fully utilized the 8GB that I have in my desktop.

If using 2 types of RAM make sure they are the exact same frequency, timing, and voltages. It is always a better option to use 1 kind of RAM but 2 should work as well

+4GB of a RAM with matching Clockspeeds ... (they [*the mobo] matches thems, anyway but can't hurt to having the same CL)
doubt your brother will ever need more than 8GB on that machine for gaming, unless he has another purpouse (rendering and whatnot)

issues with different RAM on one mobo are to my knowledge quite rare ... > buy on amazon or elsewhere you could
comfortably and sent back in the high unlikely case of incompatibility

You don't need more than 8gb for gaming atm,get 4gb DDR3

Don't mix and match, it will work but it's not the best thing.
Since you only have 2 slot, I'd upgrade to a 16gb kit and give your current ram to your brother, or if you want to save money, just get one 8gig stick so you can add another one in the future.

I suggest that your brother upgrade his pc to 16 GB ram ( 2x8 ) and sell his 4 gb ram in online stores or another available ways !

Sorry for English !

I think that I will go for the 4gb. I don't need more than 8GB right now, neither my brother. I think that's the best to do. Waste of money to have 12gb or leave the 4gb alone(i could try to sell it, but takes time).

Thanks for the points and opinions you guys gave. :)

RAM should generally run in pairs for best performance