In order to do this, we have to look at math. Yay!
what is this st00pid scarf
This is the formula for gaining experience in Generations I-IV. It seems complicated, but it really isn't.
EXP is the experience you gain.
a is a value that equals 1 when you battle a wild Pokemon or 1.5 when you battle a trainer.
b is a value that equals the base experience yield of the opposing Pokemon's species.
e is a value that equals 1, or 1.5 if one of the participating Pokemon holds a Lucky Egg.
L is a value that equals the level of the fainted Pokemon.
s is a value equal to the number of participating Pokemon that haven't fainted. This changes with EXP share.
t is a value that equals 1 when the Pokemon's OT is you, 1.5 when traded, and 1.7 when traded internationally.
Now, in order to find out which E4 gives more experience in their 2nd+ runs, we must look at each and every Pokemon the E4 (not including Cynthia/Lance) carries. But, there is one noticeable difference...
Look at HGSS, and then at Platinum. Compared to Platinum's level 65-75 Pokemon, HGSS has sad levels of 58-64. No matter what the EXP yield, level always plays a difference in this formula.
Answer? Platinum's E4 gives more EXP due to it's higher levels.
Read up on Experience. How much EXP Platinum's E4 gives.