Maximum RAM support is 8GB total or 4GB max per slot, so a 8GB module can't work. Get another 4GB module preferably the exact brand and model as your 4GB module for best compatibility... that's the best you can do for you notebook upgrade.
Gigabyte Q1542N notebook.
http://www.gigabyte.co.za/products/product-page.aspx?pid=4227#sp
http://www.gigabyte.co.za/products/product-page.aspx?pid=4227#kf
Check the specifications tab: It says;
"System Memory 2/4GB DDRIII, 2 slots (Max 8GB)"
2x4GB is obviously 8GB installed... it simple math anyone can understand so they didn't have to add "(Max 8GB)" but the fact that they did add it, is to specify that it's the maximum the notebook supports, and a 8GB module not working confirms it.