Hi Prasanthi,
Create *-1 association between site entity and workflow context object and 1-1associatio between account and site entity , now create boolean attribute in site entity, and let it be 1 usertask, suppose in parallel execution it goes to 4 approver then create same 4 site entity and put in site_workflowContext association and also put each site entity to site_approver association such that each approver has 1 site entity.
Now 1st approver approves then make that site entity boolean true, rest will be false, gradually with each approver make respective site entity boolean true
when last approver approves it make last site entity boolean true and check if all site entity for that workflowcontext object is true if yes then complete user task.