744
On the 747-400 course we were taught that to avoid transferring fluid from system #1 to #4 through the brake lines we indeed had to pressurize #4 first.
While the hydraulic systems are "separated" (by the antiskid shuttle valve) they do have a bit of common tubing, that of the brake lines downstream of the antiskid shuttle valve (which is why if there is a hydraulic leak in the brake lines, it is actually possible to lose systems 4, 1 and 2 in sequence).
The amount of fluid in the brake lines is small but the airplanes are pressurized various times each day...
MD