Wow Bam,
I can sense your frustration right through the computer screen. Each child is precious and I wish that each child was treated that way more often. If you are feeling mad at yourself for not doing more sooner - please stop blaming yourself. You did the best you knew at the time. But lots of us slip into kicking ourselves from time to time - if company helps, you have lots of it.
Private testing might give you some answers. Would the school allow admission to the gifted program based on private test scores? If so, it certianly seems worth a try.
An alternative is to get both kids involved in Regional Talent Search testing - less expensive, and if they qualify it opens up weekend and summer and year round online learning activities. (that would probably be SAT for your older daughter, and SCAT or Explore for your younger) - the idea of the talent search is to compare the whole pool of 'bright or beyond' kids to each other.
I would find that 4th grade Science/Math teacher and ask that teacher for help. Sometimes they can 'bend the rules' and allow a child to take gifted classes even if their scores are on the other side of the cutoff.
In the end, the most important thing is to try to determine what your daughter's social and educational needs are, and see if modifications are needed to meet her individual needs.
Keep posting - I'd love to hear how things are going.
Love and more Love,
Grinity