should i use all 4 ram slots
How to install memory (RAM) in your PC | PCWorld
These are the motherboard slots you insert your RAM into. ... If you’re not filling all the available RAM channels, read your motherboard’s manual to see which specific slots you should fill first.
memory - Is DDR4 RAM compatible in a DDR3 RAM slot? - Super User
In this case, my suggestions will be that you should use multi-channel if your motherboard offers 4 RAM slots or more and single-channel if your motherboard only has 2 slots, then you will be able to upgrade it later on.
Which RAM slots should I use? - Corsair
Use CPU-Z to check if you have dual rank RAM (which means you should probably leave one slot empty for each dual rank RAM stick).