An empty layout in the game without any art, sound or events. One of NW.js processes uses minimal ram here, but there's always one process (which I guess is cache) uses roughly 1.5 gb even in an empty layout.
There's two nw.exe process, the smaller one is your current game/layout.
There's a second process of nw.exe that loads all your assets as they appear in your layouts. It acts as a cache I think. The more layouts you change, or more sprites you call in, the bigger the cache becomes. Once it maxes out (loaded everything available in your game), it doesn't grow.
I'm not sure what would happen if there's not enough system ram for the cache to grow since my PCs have 12-16GB ram and my game tops out around 700mb. If yours top out at 4GB for the full game, I don't think you need to worry for PC/MAC as most low end systems have 4-8GB memory.
If I had to guess what happens when you don't have enough memory for caching, it would load from your disk, causing major stutters as big sprites with animations are called in.
Maybe Ashley can verify or explain the inner workings of this process.