The following is a picture of my brake master with ports labelled.
The top 3 I know where they go.
The bottom one (number 4) is one that I have no idea of, is one that can / should be blocked of and / or where does it go to.

Also you can see the bottom of my brown coloured bottle, it seems you cant replace them so I was thinking of making a custom one, has anyone had a go at this our sourced one ?

Thanks Gavan