Get a bid for converting the room back to original condition. That will be your future risk going forward.
This kind of thing happens all of the time. At least your seller was honest and disclosed to you.
It is very common for sellers, and sometimes their agents, to call a converted space a bedroom. They might say it qualifies because it has a closet or a door, but that is never true. Regardless of the room's design or construction it is never a bedroom until a permit is issued.
If you can find who the original builder is then maybe there was an option to have this as a fifth bedroom in their original plans. I might check with the neighbors, or maybe the planning department if any floor plans are available within that neighborhood. Depending on how long ago the homes were built would make a difference in whether the plans existed but it's a quick trip to the city of Folsom to check it out.
And as for the tax record not matching the actual home built? That's really common. You can contact the city/county and find ways to get that corrected in the records. Sometimes it makes a difference on the value assigned to the home so it's often worth doing.
Perhaps have a Local contractor take a look at it, ask them hoe much would it cost to put it back to the previous condition, and if it is possible to get a permit for it the way it is now, and the cost associated with that.
Given these two costs, you may be able to calibrate your decision easier.
All the best,