Is there any way I can adjust the output voltage of all four rails by I2C before turning on the regulator?
The MPM54304 has Multiple Time Programming (MTP) where you can save registers via I2C so that the MPM54304 will start up with these saved registers on power cycle. The output voltage is something you can program this way. Note that the part needs to be on and this can only be done 2 times.
Otherwise, you will have to adjust the output voltage of the MPM54304 via I2C once the part is on and the delay for I2C programming has occurred (about 100ms). If you do not save to MTP, the IC will not save these values on power cycle and will instead turn on with the previous defaults.
Finally, there is a version of PMIC that has all rails off at default, and the voltage can be changed before turning on the IC. Please talk to an MPS engineer for more information regarding this version of the IC.