A re-observation policy will have to be agreed by all surveys before the start of observations and depending on the two distinct definitions of a "failed exposure", as described below.
Failed exposures in the context of instrument failures and poor observing conditions are expected to be extremely rare. In the case of such an event, the fiducial re-observation strategy is to not re-observe failed fields (Observation Blocks), but rather to inform the Operations System (OpSys) that an OB failed and with what quality flags. The OpSys can then decide to re-introduce or change the specific OB. The quality flags will be provided by the Quality Control 0 (QC0). Malfunctions or failures detected by QC0 will be overall (technical) failure of the instrument or other circumstances that affect the overall usability of the OB (such as e.g. saturation or overall low signal to noise).
In 4MOST, the success of exposures is also defined in target-centric terms, i.e. has a target reached its required signal. If not, it is put back in the stack of targets to be observed. The policy of how to deal with unfinished targets in partially done fields will be defined based on scientific reasoning.