Setting up Multiple Speakers for backyard using one receiver

Hey guys,

I am doing some work in the backyard and want to wire speaker out there (I want 4 speakers total in the backyard). I want to connect these speakers to my receiver that is inside my house. This reciever is curently being used for my indoor surround setup. However the receiver i have does have a surround back speaker inputs that are currently not being used. Now that would be fine to power two speakers outside - but how do i add two more speakers for outside?

Any help would be appreciated.