The RAM (4GB) in this system is soldered to the main logic board. This system does not have any means to add additional memory. Asus made a second model in this series a X552EA which has 8GB soldered.
There is a single DDR3 slot on the back of the board. For the X551MA that I have, it was possible to remove the 4gb card and put in a DDR3 (low power, but backwards compatible) 8GB single stick. Just because you can't see it on the front of the motherboard, you should remove the screws and have a look on the back to make sure! If they have 4gb of RAM soldered on there, I would love to see a picture and get a part number for the chip(s) they use! :)
