Type 4 complexes form when pi-System Lewis bases are protonated. Type 4 Lewis acid/base complexes may be very weak Brønsted acids (for example toluene, the H+/benzyl anion complex, pKa 42) or they may be strong acids like protonated ethylene (the methyl carbenium ion) pKa 15. For more information look in the Chemogenesis
webbook sections on Lewis
acids and Lewis bases and the Lewis
acid/base interaction matrix.