Issue in setting dynamic value for dependent optionset

Hi,

I have 4 optionset fields say Department,Sector,Subdepartment,Section.Which is dependent as below,

Department->Sector->Subdepartment->Section.

Ex :HR department has 2 sector whereas other departments has only one sector.So except HR dept i need to set the sector value by default based on Department.

I tried using Business rule also using Javascript,sector value is getting assigned but subdepartment and section is locked say dependent optionset is not recognising the sector change.