Some features of the reaction pp → pp π + π + π − π − are discussed and models of production mechanisms compared with the data. Excellent agreement is found with the Reggeized multiperipheral model, when this is modified to account for the observed strong Δ(1236) and π(760) production. Baryon exchange is found to make an important contribution to the cross section. On the other hand the multiperipheral mechanism appears to suppress the importance of diffraction dissociation which is not in evidence in our reaction. Fits to a modified bremsstrahlung model are not very satisfactory but reveal the interrelation of the proton shape to the exchange mechanisms.