FWIW - being under par after a round at -2, -6, -8, or whatever is not your score. it is your how you played relative to par for the course.
If par is 72, -4 means your score is 68, not -4.
If par is 71, -4 means your score is 67, not -4
If par is 70, -4 means your score is 66, not -4
If you only play 9 holes:
If par is 36, -4 means your score is 32 and for your scoring average it's counted as 64.
If par is 35, -4 means your score is 31 and for your scoring average it's counted as 62.