I have looked on ebay for you but I can't find a separate entry for batteries, for your phone.
It could be that they are no longer available. As you say, the Vodafone site isn't very helpful.
But have a look at on this ebay page.
http://www.ebay.co.uk/sch/Mobile-Phones-Communication-/15032/i.html?_from=R40&_nkw=batteries&_dcat=20357&Brand=Motorola
Call one of the sellers on here and ask if they sell batteries for your phone.
It could be that the smart 4 mini uses the same battery as other mobiles. Take the battery out and see what its serial number is.