How do i make Hydroquina 4% in Unguentum aquosum BP-MFS? And what is the shelf life?

An ointment with hydroquinone I could not find in a formulary. Well a cream. But your excipient does not contain water. So we do not solve the hydroquinone because this causes a modification of ointment type. So you need to make the   Hydrochinon as fine as possible. You can use an antoxidans for fats like 0.1% liquid Vit E. If the ointment feels grainy then you can possibly carry the whole through the ointment roller.  

I cannot stick to this formula for a   shelf life. For creams with Hydroquinones it amounts to 4 months, if stored below 25 ° C. You may also be able to use it for the ointment, and 4 months will be sufficient taking into account that one assumes for a magistal preparation that it is used up within 2 months.