Abstract : Most of radio-frequency (RF) photo-injectors operating in the world are made of 1,5 or 2,5 cells. Although excellent qualities of electron beam have been obtained there are few cases where the extension of the number of cells could be interesting. For instance, the small accelerators with energy in the range of 10-20 MeV which are mostly based on the operation of a RF gun with a booster. One single RF gun fulfilling both functions would simplify the construction and the cost of such machines. The inherent simplicity would also ensure a better reliability. We will present 2D and 3D RF simulations of this 4,5 cells RF photo-injector. In addition we will compare through beam dynamics simulations, with the PARMELA and ASTRA codes, the performances of this gun with respect of classical design based on the couple RF gun plus booster.