Septic system was designed for 3 bedrooms but we need 4, what are my options?
There are a combination of rules for septic systems.
The size of the tank is one of the things but it is not the most important.
The drain field has more to do with the size of the system and the percolating rate or how rapidly the water from the field leaches into the ground(or not).
I have built homes where 75’ was enough for each bedroom and others took almost 200’ per bedroom (heavy clay but the soil met the minimum percolating rate) both had the same size 2 chamber tanks.
It may be as easy if you have enough property to add a distribution box on one of your existing lines in the field and create a larger field.
Check with your local building codes office (where you get the permits) they may say on an existing system no problem or if the area ok known for problems or close to a stream , creek or listed county drainage area it may not be possible.
I have turned 2 bedroom homes into 4 bedroom with this simple (but kind of yukky) addition to the existing field. I have also been told no it would require a pumped / mound system (very expensive) and they did not even really look at the lot map they also did not get the permits and extra taxes for the improvements oh well.
So it may be possible and not that difficult check with your county offices and maybe it will only be 100’ of lines (put in 200 so your home would be legal).