How to know how much current is generated?
Important points

LM3420 is a Battery Charge Controller IC.
It's to control the power delivery, not to deliver the power into the battery.

Take a look at the Absolute Maximum Ratings section on page 4. It could only source up to 20 mA of current. It is not designed to output with that much current! It is the maximum before getting damaged! 





Basic working principle

Take a look on page 10.





Example of use

This IC is intended to be used as a feedback loop control of your Boost Converter system. 
Take a look at the datasheet at 9.2.4.2 section on page 12. At first the IC will force the boost converter into constant current mode and then when the battery voltage reaches about 8.4V it will force the converter into constant voltage mode. I'll be quoting its written detailed working principle since it would be redundant for me to write it all down here.




  A switching regulator, constant-current, constant-voltage two-cell
  Li-Ion battery charging circuit is shown in Figure 17. This circuit
  provides much better efficiency, especially over a wide input voltage
  range than the lineartopologies. For a 1-A charger an LM2575-ADJ.
  switching regulator IC is used in a standard buck topology. For other
  currents, or other packages, other members of the SIMPLE SWITCHER®
  buck regulator family may be used.
  
  Circuit operation is as follows. With a discharged battery connected
  to the charger, the circuit operates as a constant current source. The
  constant-current portion of the charger is formed by the loop
  consisting of one half of the LM358 op amp along with gain setting
  resistors R3 and R4, current sensing resistor R5, and the feedback
  reference voltage of 1.23 V. Initially the LM358 output is low causing
  the output of the LM2575-ADJ to rise thus causing some charging
  current to flow into the battery. When the current reaches 1 A, it is
  sensed by resistor R5 (50 mΩ), and produces 50 mV. This 50 mV is
  amplified by the op-amps gain of 25 to produce 1.23V, which is applied
  to the feedback pin of the LM2575-ADJ to satisfy the feedback loop.
  Once the battery voltage reaches 8.4 V, the LM3420 takes over and
  begins to control the feedback pin of the LM2575-ADJ. The LM3420 now
  regulates the voltage across the battery, and the charger becomes a
  constant voltage charger. Loop compensation network R6 and C3 ensure
  stable operation of the charger circuit under both constant-current
  and constant-voltage conditions. If the input supply voltage is
  removed, diode D2 and the PNP input stage of the LM358 become reversed
  biased and disconnects the battery to ensure that the battery is not
  discharged. Diode D3 reverse biases to prevent the op-amp from sinking
  current when the charger changes to constant voltage mode.
  
  The minimum supply voltage for this charger is approximately 11 V, and
  the maximum is around 30 V (limited by the 32-V maximum operating
  voltage of the LM358). If another op-amp is substituted for the LM358,
  make sure that the input common-mode range of the op-amp extends down
  to ground so that it can accurately sense 50 mV. R1 is included to
  provide a minimum load for the switching regulator to assure that
  switch leakage current does not cause the output to rise when the
  battery is removed.




To answer your question


  I need to know how much current comes out of Pin 5.


Since you gave 4.5V in the input pin, it is below 8.4V. Thus, the output current will be almost zero, it will only outputting leakage current of a few micro-amperes or less as the datasheet stated implicitly at Electrical Characteristics Section page 6. 



Conclusions

Read the datasheet thoroughly before you use it. You'll end up destroying everything if you're not careful!
You're lucky this time you're not damaging anything.