Your typical speaker for home use is rated at 8 ohms.  The Sonos amp is designed to power a pair of 8 ohm speakers, but you can connect 2 pair of 8 ohm speakers, which presents 4 ohms to the amp.

https://support.sonos.com/en-us/article/connect-four-speakers-to-your-amp-or-connect-amp

The BB set your referring to has speakers in the set at different ohms and wired in such a way that the Sonos amp will ‘see’ 8 ohms and work just fine.  It’s not a setup that Sonos recommends, but it works.  I’ve never heard this setup myself so I can’t say it sounds good or bad.  The reviews on BB site is probably the best way to go, although no one can tell if you the BB setup sounds as good as what you have in place now.

If you intend to drive your existing 4.1 system (not sure what speakers those are or how you have 4 channels of audio with a sub) with a single Sonos amp….that’s not going to work.  Two amps bonded together would give you 4.1 capabilities (2 front channels, 2 rear  channels, and a powered sub), but that’s not typically what people want in outdoor spaces.  

Bought the 6.1 Sonance system for the backyard and have to say, the system is phenomenal.   The little Sonos Amp is plenty powerful to drive the 6 speakers and the sub for the >1500 sq ft area, but follow their instructions to daisy chain the speakers and sub or you will have an amp prob.  I ran an overkill 12/4 oxygen free 100’ run and followed their wire diagram down to using cable ties to band the wire loops at the speaker connection.  Install was a breeze.   Bottom line is that 150 watts is plenty to push sound in the backyard.  I’ve not had to push the vol to the top 90% and I like music on the louder side.  Sounds are CRISP and ULTRA CLEAR.   As to some of the comments that Sonos doesn’t recommend the setup, I find that hogwash since Sonos collaborates with Sonance and would have needed their blessing to bundle the setup since it’s factory bundled by Sonance covered by both companies warranty.  Also, I had an issue where my Samsung Terrace 75 wasn’t being read by the Sonos amp for the HDMI Arc connection and the team at Sonos tech started the call with what system I had and they were super at pointing me in the right direction to remedy.  The tech went thru my setup with me and said the calls regarding this system are typically “how to” calls and very very rarely operational issues.  Its solid…  I highly recommend it and my friends all find it to be one of the best sounding backyards they’ve experienced.