I think 4G is an overkill, it's unlikely that anything more than 1G will help. The working set itself is 35 MB -- Mongo will never need more than that.
Having said that, you should monitor memory using the linux top or free commands. As long as you are seeing enough free memory, you are good to go. When you see it nearing 80% of your memory, it's time to consider adding more RAM.
But my guess is that you'll run into CPU% issues before you hit memory issues. So, monitor total CPU usage as well -- when you see consistent 50% CPU usage, it's time to consider upgrading the CPU.