Tech specs @
http
/h10010.www1.hp.com/wwpc/pscmisc/vac/us/product_pdfs/HP_350_G1_Notebook_PC_datasheet.pdf
You can have upto 16GB of memory installed, so the options.
1. If a single 4GB unit installed, you can add a 2GB, 4GB or 8GB unit.
2. If 2x2GB units installed, you need to remove 1 and install 8GB unit.
Adding mismatched memory size although will improve Windows, you will suffer a small performance penalty over having two matching units.
My choice would be to add another 4GB if a single unit already installed.