Traktor Scratch Pro = 4 decks
Traktor Scratch Duo = 2 Decks
If your just using 2 Decks the Audio 4 will do the job just dandy.
However you have to study the purchasing options of TSP and TSD as you may want the features of TSP which are greater than in TSD and TSP is only sold with the Audio 8.
If you already have TPRO there are many upgrade options.