Night Terrors or Sleep Disorder.. What is wrong?

I have twin 4 yr old daugters. The younger one, Julia has always slept terribly even since she was born. I can count on 1 hand how many times she has slept through the night in the past 4 yrs. She goes to bed between 730-8ish Our routine is this. 6:30-7:30 is quiet time. Both girls get to pick one show while they eat their snack. After shows we read 2 books and then go potty and brush teeth and then it's bed. This as always been our schedule. Julia will usually sleep good until about 1130 or so and then for the next 3-4 hrs is crying out loud, screaming, tossing and turning. I've always gone in and either held her, rubbed her back tried to do anything to console her. (She does this every hour or so) While doing this she's usually still sleeping. She will then sleep from  usually 4 until about 630 when she gets up for the day. Sometimes she will get up and go potty but usually only once. I've talked to the dr and he says it's night terrors but I don't know.. I could see it being night terrors if it was once/week or so but every single night. I mean she's genuinly a happy child.  She's shy but only with people she doesn't know. She loves to play and is very active. I long for a full night sleep but even more so for her. Anyone every experienced this? Sorry so long.