What is the group number or name for elements between group 3 and 4 (F-block) on the periodic table?
I suppose IUPAC hasn't bothered with group numbers for the lanthanides/actinides quite yet because there would only be two elements for each group, and because there are few vertical similarities among the f-block elements. The entire row of lanthanides behaves very similarly due to the 4f orbitals being anomalously compact and not being available for bonding or ionization. The first half of the actinides is quite different to the lanthanides, as the 5f orbitals there can actually participate in chemical interactions, though the second half becomes more similar as the 5f orbitals are once again strongly trapped to the atom, this time by the very high effective nuclear charge.