MX480 is a single power zone. That means all of the power supplies are sharing the load. My guess is it will depend on how much power is allocated.
Look at the output of show chassis power and see what the allocated power is. Here is what I show for high power.
> show chassis power
PEM 0:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 171 W (zone 0, 3 A at 57 V, 8% of capacity)
PEM 1:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 228 W (zone 0, 4 A at 57 V, 11% of capacity)
PEM 2:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 171 W (zone 0, 3 A at 57 V, 8% of capacity)
PEM 3:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 348 W (zone 0, 6 A at 58 V, 16% of capacity)
System:
Zone 0:
Capacity: 4100 W (maximum 4100 W)
Allocated power: 1441 W (2659 W remaining)
Actual usage: 918 W
Total system capacity: 4100 W (maximum 4100 W)
Total remaining power: 2659 W
Since my allocated power is only 1441 W, and two PEMs in high power mode provide 4100 W, I take that to mean the system can drop two PEMs without a problem. It should be able to run on a single PEM since the high power PEM capacity of 2050 W is greater than the 1441 W allocated. Unfortunately the MX480 is live so I can't confirm if that is true or not. Based on what I recall of the MX960 behavior when it was a single zone (before the high capacity power supplies), I'm fairly confident that it will behave as I expect.
MX960 with the high capacity PEMs is zoned, so where A & B power go matters. PEM 0 and 2 support a zone, and PEM 1 and 3 support a zone. So on the MX960 using A & B power, it needs to be connected A, B, A, B. When the MX960 was running with a single power zone which ones were A or B didn't really matter. Same should be true for the MX480 since it is also a single power zone.
Remember, this is an educated response, not a definitive answer. For a definitive answer, check with JTAC and / or test yourself.
-Chad