The ASQ developers used 39 weeks as a full-term pregnancy in their research and data analyses. In your example, the child's gestational age is rounded up to 35 weeks. 35 weeks is subtracted from 39 weeks (a full-term pregnancy) so the child is 4 weeks premature.
However, if your program uses 40 weeks as a full-term pregnancy, you can continue to do so while using ASQ. Using 40 weeks as the standard, the child would be considered 5 weeks premature (40 weeks minus 35 weeks).