The 4 GB of memory that is installed is typically a pair of 2 GB DIMM modules (so that it will run in a dual channel configuration). Therefore, you'd need two 4 GB DIMM modules to upgrade to 8 GB of memory.
You can install an odd amount of memory (for example, 6 GB) by installing one 4 GB DIMM module and leaving an existing 2 GB DIMM module installed, however I wouldn't recommend it.
You can check to see if this applies to your MacBook Pro by clicking the Apple menu, click on About this Mac, and then click on the "More Info..." button. In the System Profiler window on the left hand side choose "Memory". It will then list the available slots and what is currently installed along with other details.
Hope that helps, have a great day. 🙂