Speaker and Amp - What do I need to make this work??

I moved into a home where they had previously installed by Bose 4 outdoor speakers and 4 indoor surround sound speakers into the ceiling. They all run at 4-8 ohms ea... I know that its possible to run 4 speakers onto an amp, but the speakers HAVE to be 8 Ohms to do this (my understanding).

My problem is whats the most cost effective solution while still maintaining a good sounding set-up? Do I have to buy 2 Amps for indoors and 2 for outdoors (with only 2 speakers connecting to each)? or am in the clear in connecting 4 to each since the speakers have a range of ohms from 4-8?

The Sonos Amp works just fine to power 4 speakers (8 ohms as you stated), and this is exactly what I do in my backyard. It's certainly worth trying out to see if it meets your needs. You can buy a second amp later on if you want to.

Inside, you can only use a single amp to power surround speakers in a Sonos HT setup (5.1). Surround sound typically doesn't require much, so there is no reason this won't be good as far as power goes. However, your surround speakers will be limited to rear left and right channels, so your left and right speakers will be sharing the same audio, If you're expecting to get 4 separate surround channels (as typical in a 7.1 setup) then Sonos is not the right HT setup for you.

You may want to consider getting a 7.1 AV receciver from a different company to power your surround speakers, along with 3 new speakers for the front. You can make this part of the Sonos system by adding a Sonos Connect. There are Onkyo/Pioneer AVRs that are designed to work with Sonos for easier control.

My preference would be to just buy all sonos, but since the indoors are already mounted and the sonos have to be plugged in, I'd need an installer/electrician to come out. I'd prefer to avoid that since I'd have the additional cost of needing to buy new speakers as well.

That shouldn't be necessary.