RAM issues

[Leazlan]

Hi guys, I'm pretty new here so not to sure what to do..

Anyways here goes, I built my own PC last week

GPU - MSI Gtx 970 4gb Twin Frozr
CPU - i7 4790k
Motherboard - MSI z97 gaming 7
PSU - Antec 850w High current gamer
RAM - Corsair Vengeance 1866mhz 4x8gb
SSD - Scandisk 256gb (used for boot up and applications)
HDD - 1.5tb Toshiba
OS - Windows 8.1 (got it from a friend)

So everything works just fine when assembled except my PC screen was just black when I switched it on so I took out two of the ram modules and then PC switched on just fine. Why can't I use all four of the ram modules? I'm pretty sure my mobo can handle up to 32gb.. I will swap out the two currently in with the ones I took out to see if the ram is maybe faulty but if it isn't why isn't it working?

Thanks for the assistance.