Depends on a few things. Where I live, septic systems are sized according to # of bedrooms. So you can't have 4 bedrooms if the septic is sized for 3.
Also, each bedroom must have an "egress" window as a secondary emergency escape. If the 4th bedroom doesn't have this, it can't be called a bedroom. Egress windows have specific requirements as to width, height, sq. ft. of glazing, sill height etc.
Where I live a bedroom also isn't a bedroom unless it is at least 120 sq. ft. and has a closet.
Beyond that it doesn't matter, what you built is a partition wall and usually doesn't require a permit, except maybe for the electrical work.