Your earning depends solely upon the quality of hubs that you are publishing. Hubs that are informative and properly structured, attract potential readers.
The amount you are earning is pretty good considering your experience of 4 months in Hubpages. However if your traffic is 500-1000 on an average, and your Hubs are good enough to be qualified as featured, you can easily earn up to $5. Again this depends on the topic that you have chosen.
I suggest you to keep on publishing more hubs that will definitely increase your hubpages earning.