why windows don't use all availabel ram?

this problem is very bad you can be have 12GB ram but windows use just 4 GB now how to slove it?

firstly if you use 32 bit based system you need to use 64 bit because 32 bit can just read 4 GB of ram

secondly all windows version have limit in how much ram can read you can view it here

now if you have 64 bit system and windows 7 ultimate version (192 GB limit) and have 12 GB ram but windows use just 3.96 like me what to do?
windows use just 3.96 because Peace of the pc like cart graphic and other peaces use part of ram and to make sure that will stop go it click windows logo and R to open run window then type msconfig click ok go to boot> advanced option click ok maximum memory and save
now restart windows now must use the all  available ram
but if the problem steel in your pc try use other ram and pet it in first place in motherboard and the old ram in 2 place
ok now i'll be hope if this help you :)