TeV scale unification in four dimensions versus extra dimensions

The gauge coupling constant unification at the low (O(TeV)) scale can be obtained just in four dimensions, without the help of the power like renormalization group evolution in extra dimensions, due to the presence of some extra particle states at intermediate scales. We show explicit examples of such extra states in the range of 100 GeV – 1 TeV which can easily be observed on future colliders and can have an important impact on the particle phenomenology. In particular, we show that the presence of a single colourless Dirac Fermion with the weak isospin T=3 and the hypercharge Y=8 with the mass of few hundred GeV would lead to the gauge coupling unification in the TeV range. The problems of the low scale grand unification and proton stability are also discussed.