That's a tiny database size, would fit entirely into memory, but according to the docs: "Because MongoDB uses a thread per connection model, each database connection also will need up to 1MB of RAM, whether active or idle." So how big is your pool?
I think 4GB is probably an order of magnitude more than you need right now, but with 1k concurrent users, it sounds like the size of your db is going to grow significantly? Also depends on how you're able to utilize index inserts, which will depend on the sequentiality of your index ids.
How much RAM do you have now? How much is Mongo using? How much has it grown as your app/db size/indexes have grown? What's your performance like now?