install 2 dimms 4GB with 2 dimms 2GB on W510

[prince_kassad14]

hi guys
I want to ask about what will happen if i installed 2 x 4GB with 2 x 2GB to get total 12GB of memory with mixed brands .
firstly is there is any problem in this case ?
is there is any performance issues may i encounter in the system ?
is it must be installed in pair or no problem ?
please guys i need help
by the way sure all memory will be same speed PC3-8500 1066