The bond length in any compound is dependent on the hybridisation of the central atom. Boron in BF3 is sp2 hybridised which means that the s-character is 33% and therefore, the bond length is shorter. Also due to similar size of both atoms and vacant p-orbital of B, a pp-pp back bonding from F to B occurs causes partial double bond character. This further decreases the bond length of B — F. In BF4–, the hybridisation of B is sp3 which means that the s-character is 25% and therefore, a longer bond length.