Having a steady 400% yearly growth is awesome, this could really turn into something big in a few years.
Btw, how much time do you spend on developing vs marketing?
Hard to say how much time is spent developing vs marketing. Probably something like 90% design, development, user support, and 10% marketing. Where marketing means writing blog posts and engaging on social media (mainly reddit, facebook, twitter).
I tried some paid advertising experiments a couple of years ago but they didn't work well at all. May try another experiment this year now that the product is more polished and has a higher value.
Regarding marketing we are in a similar spot. Only 10% of time goes to marketing and paid advertising doesn't work. I recommend reading Traction (http://tractionbook.com/), it gives you a good model on how to go about growing your company.
Good stuff with your software and I enjoyed reading your post. You need to increase your pricing though - and it seems to me you haven't fully figured out the true value of your product, so that you can "value price" it.
Email me if you'd like help putting the value proposition together. (My contact details are in my HN profile). I'm by no means an expert in what you do but I've been in sales for 15 years... so can probably still help you a bit.