Assign task in Round Robin Fashion

We have been using flowable (open source version) to design our Business Process workflow. I need to assign user (belonging to same candidate group) to a task in round robin fashion. I don’t want to use any external system/ storage to store any information. I also don’t think process/ task variables can be used to achieve what i want because they are instance level variables. Can flowable provide some global variables, generate a sequence or maintain anything in its storage.
Scenario : Leave request workflow, where leave will request will be created by employee, it will be sent first to validator, then to manager and finally to senior manager. There is a candidate group as validator which have 4 users. The validator validates the leave request and transfer it to Manager/ revert with comments to initiator. First employee came instantiated a process. Validation Task is assigned to V1 and he takes necessary actions. Now for next process instantiated v2 will be assigned. then v3, v4 and then v1 will be assigned.