Why worry about 'assigning' roles at all? In a team of 4, with a clear vision of what it is you wish to develop, this should be a no brainer. Collectively determine what needs to be done, then in what order it needs to be done. For each sprint, each member steps up and takes a piece of the pie. For non-development tasks (i.e., documentation, revision control, back-log management, etc.) determine who in the group is either A. most qualified or B. least objectionable to fulfilling each task. The most anal attentive to detail member should naturally handle revision control. The best communicator should handle documentation. The most organized should manage the back-log. If everyone on the team have similar traits and personalities, then change roles every sprint until you find what works best. Just my $.02