I would be weary of the advice you received about the septic. I believe you will need to speak to the Environmental Health Dept in your county. To add a bedroom, you will most likely be required to reperk the lot to determine if the soil would be suitable to accept a four bedroom home, and most likely to determine a future repair area in case it does fail. Without this step and the proper permits, you could be opening a whole slew of problems when you decide to sell.
If you are torn about your options to sell or stay, to make the additions or not, or just want a little more advice, I am happy to schedule a time to discussyour options further. You can contact me through Trulia and I will follow up with an e-mail or phone call, whichever you are most comfortable with.