Hi,
Your laptop supports up to 8GB of RAM but it requires a specific model of RAM in order for it to work. Your laptop supports a DDR3 1600Mhz SDRAM 2GB/4GB or 1x 2 X DIMM Dual Channel DDR3 1333/1066/800 Mhz.
Here's the complete technical specification of your laptop.
https
/www.asus.com/ph/Notebooks/ASUS_VivoBook_S400CA/specifications/