From what you've described, she can count past four - she's just leaving out one number consistently. This happened with 2 out of 3 of my kids (not 4, but one number consistently left out), and has also happened with a lot of the other young children I've known. In each case it was just a phase. One of my 2 who did this is dyslexic, but looking at her overall development I don't think leaving out the number phase had anything to do with dyslexia, I think she just didn't feel it was important to include 18
If you're seeing other things that seem odd, then talk to your ped about it and perhaps pursue a further look. I do believe that listening to your gut feeling is extremely important (and I have two 2e kiddos, in both cases wish I'd had enough obvious clues to test when they were younger), but the leaving out one number is something that I've seen happen over and over again with very young typically developing children.
OTOH, I'm just a parent, not a professional, so take my advice with a huge grain of salt
Best wishes,
polarbear
