[en] Evaporation residues from the fusion reactions ⁴⁸Ca+^206,208Pb and ⁵⁰Ti+^207,208Pb, ²⁰⁹Bi were investigated using the velocity filter SHIP. Four new isotopes ^258,257105 and ^254,253Lr were identified and α decay of ²⁵⁵104 and ²⁵⁶104 was observed for the first time. A hindrance to complete fusion by a factor of about 30 at bombarding energies close to the classical barrier was deduced from a measured excitation function for ⁵⁰Ti+²⁰⁸Pb. The mass extents for ²⁵⁶104 and ²⁶⁰106, obtained from their α-decay energies, were used to determine the ground-state shell effects and a semi-empirical description of the half-life T_SF for even-even nuclei. The abrupt decrease in T_SF for elements from nobelium to element 104 was found to be correlated with a decrease in the fission barrier width, which we regard as confirmation of the expected drop in the outer fission barrier to below the ground state. The total kinetic energies released in SF of ²⁵⁶104 and ²⁵⁸104 were measured and found to be 207±13 MeV and 220±15 MeV respectively. These values agree with those predicted from empirical systematics. (orig.)