Hi

No, additional subject is not your 4th subject rather additional subject is your 6 subject in your board examination.

For example,

I have done my 12th from CBSE board and in my board examination I had five compulsory subjects, I was a PCMB student i. e I had physics chemistry Mathematics and Biology and English was my 5th subject and all these five subjects were compulsory subjects. For the 6th /additional subject I had chosen Hindustani vocal music.

Actually, It completely depends upon you which subject you want to have as your additional subject and also depends upon the subjects which your school offers for additional subject.

Moreover,

As suggested by the name it is not mandatory to study additional or 6th subject . You may not opt for additional subject and study only 5  compulsory subjects.

I hope that you find this helpful

Thankyou