Menadiol is an organic compound with the formula C6H4(COH)2(CH)(CH3).  It is formally a derivative of p-hydroquinone. The name vitamin K4 can refer to:
 specifically this compound,
 its various esters, e.g. 
 menadiol diacetate (acetomenaphthone),
 menadiol dibutyrate,
 menadiol dimalonate, or
 its various salts, like
 menadiol sodium diphosphate (Kappadione)
 menadiol sodium disulfate.

Menadiol sodium diphosphate is approved in the UK for treatment and prevention of haemorrhage, mainly in obstructive jaundice (before and after surgery).

Menadiol is probably naturally produced by reduction of menadione ("vitamin K3"; see ) as an intermediate in the conversion from K3 to MK-4. It can be oxidized in experimental conditions back to menadione.

References

Naphthols
Vitamin K