1x Compact 1/8 NPT to 4mm Straight Hose Connector for boost connections. 
 
These compact hose connectors have a 1/8 NPT thread and are best suited to approx 3-4mm I/D boost hoses.
 
The very same fittings are used by Turbosmart, AEM, Haltech, Vipec, Gizzmo, Autronic etc but ours are at a fraction of the price and available from stock.