Can some one pls help us out asap?

We constructed a new house in June 2009 and according to plan, the septic tank is 4000 liters. Its got filled more than four times in the last 6 months. The house is in Madambakkam, East Tambaram.

I am unable to find out a solution. The engineer who constructed the house says:

1. The soil is rocky so theres no absorption of water.
2. The dimensions of the tank are 7*7*4 feet, Is this tank 4000 liters in capacity, with 9 inches wall around.
3. The tenant says, the bottom of the septic tank looks like cement and has no gravel. When asked the eng. he says, Its some lining and gravels are only for soak pits and not for septic tanks. They are only 4 people in the house, 2 adults and 2 children. They say, they do not pour any water into the latrine other than for latrine use.
4. Its such a nuisance to empty the septic tank once in a month.
5. Eng says there are 3 soak pits, one for each bathroom and one for the kitchen, So its only the water from the latrines that flow into the septic tank.

We have left 3 feet * 55 feet in the sides and the engineer says, he did not connect the septic tank to any other soak pit as theres no space. 

Of all things, he JUST DOES NOT want to come and inspect the septic tank. Also, if we wish to construct another septic tank, we have 2 places left. One at the back side of the house, which again is 2.5 feet width and 20 feet long and another in the sides, which is 3 feet * 55 feet, Is it possibel to construct another septic tank in these areas? Or can we transform our water sump to a septic tank?

Can someone pls ple help us asap? We are really tensed over this issue and live in another state and are unable to find a solution for this.