If I already have four Fire Energy cards in my Discard Pile, can I use Superior Energy Retrieval to discard two Fire Energy cards from my hand and retrieve the other four from my Discard Pile?
Yes, you may retrieve the other four. The restriction is you cannot return any of the cards to your hand that you used to pay this card's cost.
Source: Plasma Freeze FAQ (2013-05-16), TPCi Rules Team (2013-05-16)

Relevant cards

Note: All applicable cards may not be listed below.
BW9_103