I was going to make a comment that 4gb should be plenty but then I looked at the store page and saw this under the recommended specs, Memory: 6 GB RAM
Restart and check your system after it has reached steady state (bootup is finished and everything has loaded, but before Windows/defrag/av scan maintenance tasks start), assuming 5 minutes. Open task manager and see how much memory your using, I would hope it is less than 1.7gb. If not you might need to disable some items that you don’t need starting up or do a clean boot and check to see if Black Mesa continues to have a memory issue. https://support.microsoft.com/en-us/kb/929135