tinyrice, I don't know what kind of 747 Classic you have operated, but all the ones I have in my logbook; i.e -100,-200,-300 with various engines, P&W, RR, CF6, were all the same. ADP operation depended on switch position. In Auto, pump runs when system demand causes EDP output pressure to fall below 2,600 psi. Once started, pump will run for at least ten seconds (or as long as system demand prevails). In off position,pneumatic supply is shut off and pump will not operate. With switch in continuous posn. pump will operate continuously.
I don't know about the -400, they don't have flight engineers on them but I shouldn't think the hydraulic system would differ much from the classic.