I only beat the E4 in Platinum 2 times. And I was so shocked that even though I have my Pokemon in LVLs ranging in 60-65, I was so shocked that Cynthia was still 10 levels more than me. Though, I know that E4 Pokemon's LVL would be higher, I didn't know it would be after you get the National Dex.
In Pearl, countless of times since E4's lvl hadn't change a bit.