Alternatively, it's a lot cheaper to DIY:
http://www.audiomulch.com/midipic/
And even simpler & cheaper, you could use the joystick port of
your PC / soundcard, to connect 4 pot meters and 4 buttons. For
the price of 1 joystick connector, a bit of cable, the pot meters
and the buttons and a case.
(And I guess a lot of people have 2 joystick ports, one integrated
on the mainboard and one on the soundcard, which makes this 8)
http://www.epanorama.net/documents/joystick/pc_circuits.html
(see 'Connect other circuits to PC joystick port')
Maarten