Why does boron trifluoride form boron tetrafluoride ion instead of boron pentafluoride ion?
As you point out, the $\ce{B}$ in $\ce{BF_3}$ is sharing a total of 6 electrons, 3 from the boron and 3 from the three fluorines.  Now add a fluoride anion that brings 2 more electrons to share.  This now makes for a total of 8 electrons around the boron.