A tetrafluoride anion $\mathrm{XF}{}_4{}^{-}$ is formed by a group 17 element X. Using VSEPR theory, its shape is

Group 17 atom X has 7 valence electrons. Electron pairs = $(7+4+1)/2=6$, of which 4 are bond pairs and 2 are lone pairs. Octahedral arrangement with trans lone pairs gives a square planar ion.