Racers noticed that if they charged NiMhs at higher and higher charge rates (5amps, 6amps and more), the pack would dump quicker on the track - it would give them more amps. This is fine for a 4 or 5min heat race and batts that were treated this way were tossed after a short while or demoted to practice packs when they quit showing that extra. These were matched abd banalced race quality cells, much more expewnsive than the cells in your pack.

I don't think you'll notice a diff between charging at 2 vs 4 except it will charge a little quicker at 4 amps. Just make sure they're not getting so hot you can't hold them. If they do, try a lower rate, you'll damage them with high temps.