I couldn't tell whether the gym leader theme or the elite four theme was used, since they sounded the exact same except one was supposedly a semitone lower than the other. Please don't question my curiosity.
Yes I agree the theme sounds similar. The track you are talking about is actually a mixture of the E4 and gym leader theme, I just listened to the tracks. It sounds like a remix of both the leader and e4 theme.
Source: https://www.youtube.com/watch?v=c_MavjffFRA (Elite 4 theme), https://www.youtube.com/watch?v=qiCpFkybzeI (Gym Leader) I listened to all of these.
After listening to those two listen to this https://www.youtube.com/watch?v=DNZI7at2IBk