The calculation isn't too complex. The amount of EVs you need to get one stat point is equal to 400 divided by the Pokemon's level, aside from rounding and other quirks.
So that this doesn't confuse anybody: no, it's not a problem you EV trained your Pokemon at a low level. Your Pokemon's stats -- their EVs accounted for -- are constantly recalculated. So if you train 8 EVs for one stat point at level 50 (for example), those EVs will later show for two stat points when you level up to 100.
Make sure you fully EV train before you reach level 100 in Gen 4, though. Due to the recalculation mechanics described in the answer, getting EVs to translate to stat points is a bit tricky when you can't level up anymore. This is not a problem for modern games.