# Does a [B4O8]4- anion really exist?

I am currently collecting information for an abstract and therefore I wanted to discuss different $$\ce{[BO3]^3-}$$ and $$\ce{[BO4]^5-}$$ connectivities. I also found a book on crystallography and the following passage

(Summarized translation of the original German text in [1])

When a finite connection of $$\ce{[BO3]^3-}$$ units is considered group borates form. [...] $$\ce{[B2O5]^4-}$$ (via vertices), $$\ce{[B2O4]^2-}$$ (via edges) or for example $$\ce{[B4O8]^{4-}}$$ (four-membered ring). [...]

It's about the last example that I cannot really find any other mention in literature. Although it looks okay if you consider vertex-connected oxoborate units forming a $$\ce{[B4O8]^{4-}}$$ anion, I cannot seem to find any example in literature. Even the book doesn't give any example.

I found some minerals for example $$\ce{Ca2B4O8}$$ but it's also described as $$\ce{CaB2(OH)8}$$. There are examples with $$\ce{[B4O8]^{4-}}$$ but they always contain additional $$\ce{(OH)-}$$ ligands.

The any other explanation I could think of would be $$\ce{[B4O7]^{2-}}$$. That would have a different structure (they also sketched their molecule anion and the got the charges correct so I don't think $$\ce{[B4O8]^{4-}}$$ is typing mistake). So, did anyone else find any example on a $$\ce{[B4O8]^{4-}}$$? I would usually check the Gmelin but due to the virus spread our library is currently closed.

[1] W. Kleber, H.-J. Bautsch, J. Bohm, Einführung in die Kristallographie, 19th edition, Oldenbourg Verlag, Munich, 2010.

Edit: I also did a quick DFT, hybrid functional B3LYP with Def2-SVP on the anion and according to my bond analysis (don't know if you can trust that for a tetra-anion here) all $$\ce{B-O}$$ bonds inside the ring have a bond order of 1.x while the external $$\ce{B-O}$$ bonds have a double bond character (I guess it's treating them as $$\ce{B=O}$$ rather than with charges here). But in theory that thing should be stable to me.