hotends.com. As with the Mk III-B, a cooling fan will be highly recommended for printing with PLA. Printing, with ABS, will not be a problem.
Until more information is available, the shorter version, with the 36.5mm nozzle holder will still be experimental.
A Mk IV, with an aluminum nozzle, will be made; but, will probably not be sold regularly as the available sizes of raw aluminum bar stock require additional machining in order to manufacture the J-Head nozzle. This, of course, would increase the price. In addition, for best results, the aluminum nozzles should be insulated.