I want to know the maximal usage current of the 5V Pin 4. If you know where to look it up in the technical documentation please let me know. I have a Raspberry Pie B+.
In normal use it would be your 5V power supply maximum rated amperage less the amount of power used by the Pi and other peripherals.
These are all unknowns so your question as posed is unanswerable.
However the Pi's PCB traces may well act like a fuse (and pop) if you try to send 3 or 4 amps through them from the microUSB to the 5V pins.
EDITED TO ADD:
I forgot the 2 amp polyfuse at the microUSB. So two amps is the effective maximum you can feed into a B+ via the microUSB.