Increasing space between staves - how to?

[po_ubl]

Hi,

I have written a score with 4 staves (please see below) and I want to increase 
the space between the 3rd and the 4th stave, and *only* between the 3rd and the 
4th stave.

How could I do that ?

Thanks.

\score {
\new StaffGroup <<
\new Staff << \global \Clarino >>
\new Staff << \global \Quinta >>
\new Staff << \global \Alto >>
\new Staff << \global \VulganoEtBasso >>
>>
\layout { }
%\midi { }
}