Could very early ultrasound (5 weeks) be not accurate? it showed only 4 weeks, what doesn't match my ovultation. at 12 week it was correct...

That's correct: The accuracy is plus or minus a week or so. The prenatal ultrasound dating scan done at 8-11 weeks is more accurate. That means when the doctor says, "The scan dates your pregnancy at 9 weeks," he is actually saying, "The scan dates your pregnancy at 8 to 10 weeks." In almost all normal pregnancies, the exact dates are unimportant.