I like this question.
I'm rather c ertain they meant one per..
but going by the wording. I think it would give you a d4 per type.
Benefit(s): As a swift action while wielding a magic weapon with a weapon special ability that deals extra acid, cold, fire, electricity, or sonic damage on a hit, you deal an additional 1d4 points of damage of the same type with each hit you make with that weapon for 1 round.
The wording of the listing of types. and then later "of the same type" reads like it adds bonuses per type. It does not tell you to choose anything. If X energy then D4 bonus.
well. Thats my thoughts anyway.
I'm sure someone will have a better idea.