Not the cheapest ever, but it seems the 4 series has been discontinued in favour of the new 5 series. So if you want another Charge 4 to connect to one you've already got, this is a decent deal. Note that the new 5 series won't connect to the 4 series.
Comes in 3 colours. Free delivery.
USB-C charging and also functions as a power bank. I've had one of these for a while and really like it.
These are better IMO than the series 5. The series 5 doesn't have a cover fro the USB-C port. I realise that the ports on the 5 are water resistant without a cover, yet the cover does help dust, sand etc get in when using outside.
My understanding is that to connect them, it must be run through the JBL app. If that's the case, its not worth it for me.