I'm a little confused - did she get 99% compared to 4th graders or 8th graders? Anyway if she got a 19 as a 4th grader, be ready to be open to the possibility that either she is more unusual than your thought or the range of average is quite lower than you thought.
Grinity, are you saying that a 19 on the English section is the 99th percentile for 8th graders? I didn't think a 19 was that high. My vague recall is that the 99th for 8th graders on the English section was somewhere around a 24. My oldest got a 19 or 20 on the English part of the Explore when she was 9 and I'm pretty sure that wasn't the 99th percentile.
Either way, I'd tend to agree that a 4th grader who is in the 19-ish range on the English part of the Explore (especially if her reading and writing skills are similarly strong and not just language mechanics), is going to be more HG in that area than the typical gifted kid and need acceleration even within a GT program.