Hitec 425BB current requirements:
NO LOAD (idling current, just sitting there), 8ma;
UNDER LOAD (moving) 150ma, so running 4 servos, you need at a least 5-6vdc, 600ma wallwart, thats th' minimum requirement, at least 1amp would be wot I would run mine at, with 2amp wallwart even better.
What I'm going using: a computer power supply to run the servos. Most supplies provide regulated 5dv, at up to 20amps, so you could run up to twenty-plus, 4-servo skulls. I picked up a 200watt supply fer $18. I connect an extension cord to the supply's 5volt output, run that across the yard to my pirate ship where abt 12 pirates will be (abt 48 servos), and not worry abt current loading. My single pirates will be powered by single 6v, 2.5-3.7amp wallwarts (have several) so no worries there either. Again, each Hitec 425BB under load requires 150milliamps, so each 4-servo skull should be run by at least a 5-6vdc, 600milliamp wallwart.
Hope those numbers help ya figure if yer currently, under-current
Capt. Jack