setting the impidence level

[cembros]

i just got my polk lsi15's and lsic today and im attempting to set them up with my sony da3300es. I know the speakers have a nominal impidence of 4ohms and i can set my receiver to that output, but the problem is my rear speakers are low end polk bookshelves that have an impidence level of 8 ohms. My question is if i set the receiver to 4 ohms will this damage my rear speakers, and visa versa, if i set the receiver to 8ohms will this damage my front fronts and center??