You will need an amplifier to drive them. How much power depends on what they can handle and your application.
Something like this would work https/www.amazon.com/Lepai-LP7498EA-200W-Digital-Amplifier-Bluetooth/dp/B00FJWKZP4
If you want to control the volume of each pair separately and turn then off then an impedance matching volume control for each pair would give you that functionality.
You can also use any 2 channel receiver with speaker A+B. You can add a bluetooth receiver to any audio receiver regardless of age. http/www.bestbuy.com/site/insignia-200w-2-0-ch-stereo-receiver-black/4156007.p?skuId=4156007
If your budget is higher then Sonos would work great. A ConnectAmp could play all four and instead of just streaming from your phone the Sonos App lets you phone act as a remote for Sonos so it doesn't need to drain your battery while music is playing.
You could add a second Connect Amp and have two separate zones that can play different music at the same time.
You might like to consider small a solar panel for each speaker. Bluetooth won't be a problem if the speakers have tha facility. In my office I use an old phone with no SIM but a SdCard full of music and that plays to a Bluetooth speaker inside an fifty year old transistor radio case. The phone and the speaker are powered from the same dual USB electric plug.