I've been stuttering since i was 4 aswell. Fast forward, I am pursuing engineering and currently in my 4th year and I have to give vivas every semester. Stuttering will have absolutely zero effect on your grades in viva. All that matters is what you know about the question asked and how you are answering it. No matter how much you stutter, the teacher will not judge you upon that. I actually get more marks in viva than my normal friends all the time. So, chill.