Why are carbon atoms of glucose considered chiral?

A chiral carbon atom is defined as



> 
> "a carbon atom with four different groups attached to it."
> 
> 
> 


I am aware that the open-chain form of glucose has four chiral carbon atoms.


However, I do not undertstand why that is, since the alleged chiral atoms do not have **four** ***different*** substituents: For example, the 3rd carbon atom is bound to one hydrogen atom, one hydroxylgroup and **two** neighbouring carbon atoms.


[![enter image description here](https://i.stack.imgur.com/RHIiA.png)](https://i.stack.imgur.com/RHIiA.png)


