[en] The model of a fourth generation is currently not excluded by theoretical arguments. In this thesis a search for a fourth generation down-type quark is presented with the decay hypothesis: pp→b' anti b'+X→t+W^-+ anti t+W⁺+X→b anti b+W⁺+W^-+W⁺+W^-+X. The search is performed with an integrated luminosity of 2.05 fb^-1, obtained with the ATLAS experiment at the Large Hadron Collider running proton-proton collisions at a center-of-mass energy of √(s)=7 TeV. Events for the analysis are selected with a search signature of two same-sign charged leptons (e or μ) in the final state, which is a rare Standard Model signature. Beside using the standard event selection from ATLAS top analyses, the event selection criteria are optimized with a multivariate analysis in order to maximize the significance. Furthermore, systematic uncertainties are studied coming from the parton distribution function as well as the initial and final state radiation. With the selected events in the signal region of two same-sign leptons, the signal is extracted via a counting method. The extracted signal is used for the determination of the mass limit of the fourth generation down-type quark with a modified frequentist method. Assuming a branching ratio of 100% for the decay b'→t+W, b' masses below 461 GeV are excluded at 95% confidence level.