Finally managed to get an old Ripjaws module and as expected my computer managed it seamlessly, now finally with 8gb of RAM. Thanks to all who helped out!
As my computer has already a few years it isn’t compatible with some newer models of this same memory, I tested it myself, I got already a module fabricated on Jun 2014 that works fine but a few weeks back I bought one on a local store from Oct 2014 that didn’t work on my computer and I had to return it.
You can check the production date on the module warranty sticker.
As many have suggested this seems due to the chipset distribution configuration, on newer models it seems that manufacturers are using high density chipsets of at least 512MB, for this reason some old motherboards are unable to use this modern chipsets and bound to use module with 256MB chipsets. Of course, using less chipsets is an advantage to reduce manufacturing costs.
For this reason I am searching actively on Ebay for a Ripjaws DDR3 module for a model of similar dates of Jun 2014 or with 256MB chipsets, if you are a store or a particular and happen to have one model and willing to send to Spain please let me know. Thanks in advance!