This laptop has 2 SO-DIMM slots.
One of them has a 4GB module fitted in the factory. The other is empty.
The maximum RAM size, for each slot, is 8 GB.
So you can add a 8 GB module for a total of 12 GB.
And you can replace the current 4GB module with a 8 GB module giving you 16 GB total.
Having 2 identical RAM modules allows use of them in dual-channel mode which is marginally faster (usually not more than 3%-5%) than single-channel.
Having more RAM is usually much more beneficial to Windows/applications than the small speed benefit of dual-channel versus single-channel, so I would not worry about using a mixed RAM configuration.
Please note: Most HP laptops are quite picky and will NOT work if the RAM isn't certified. Check the HP web-site for compatible RAM modules before you buy.