Amperage rating indicates the maximum total amperage the power supply can draw (provide). It is not like voltage where the equipment needs a range to operate. That is, voltage is pushed rather than drawn like current. Thus, when the power supply rating states 4A, it indicates it can provide a maximum of 4A, with every equipment connected to the power supply drawing(pulling) some of this current. Let`s say we had a PP6 with an IMU-ISA-100C,
the maximum amperage required by the IMU-ISA-100C is 1.4 A
the maximum amperage required by the PP6 is less than 0.5 A
Thus with a 4A power supply, you still have about 0.9 Amps to spare.