I have 2 dys and 2 non-dys (the oldest ones--I never applied for them). I applied for the #3 kid because he was sooo out there. Then it was easy to apply for #4. I explained to the older kids that they were just as bright, but didn't have the same needs as #3. They can recognize it. There was some rivalry between #2 and #3, but now that they're in different schools, it's abated.
Breakaway4 -- I just want to note that when I had #2 tested I was really nervous because he didn't seem as advanced as #1 (he didn't speak until he was 3.5 yr old, he didn't read as much, he was very creative). He turned out to score 13 pts higher IQ than #1. Part of why I went on to test all of them was because I needed to understand them--clearly I had misjudged.