This is 4mm OD brass tube very nice tube BRAND NEW with a wall thickness of 0.45mm, The great thing about this tubing that the next size up (5mm) fits the 4mm inside it  and this is the same for all the tubing by K & S. All the tubing the size below fits inside each other.

This is for a 300mm long length posted same day as purchase next day delivery please check my feedback.