Hi Rui,
My colleague, Alan Lofft, wrote an excellent Axiom article on the subject . . .

Easy Answers to Confusing Specs: Sorting Out "Impedance"

" . . .you can connect speakers with different impedances (8 ohms, 6 ohms, 4 ohms) to an AV receiver with no negative effects so long as the impedance of any of your speakers doesn't go below 4 ohms. . . "


jc