Welcome Emanuel!
Kirkham will have the 4HU you need as will Shelby American. Another and much cheaper route may be to use one from a Jag as you have found but it doesn't have the right mounting's in the casting. Nick Acton sells a bracket to adapt the jag unit to the Cobra chassis.
Kirkham Motorsports
http://www.actoncobra.com/
http://www.shelbyamerican.com/index.asp
Hope that helps!
Larry