Reduce voltage from 12V to 3.7V/5.4A
Yes you would use a switching Step Down DC/DC converter.  There are many available.  Search for a Buck converter.  Places like Digikey, Amazon and eBay will definitely have them.  They are available as a voltage or current source.  
If a current source is needed, then the voltage just needs to be higher than 3.7V and be spec'd to have a minimum a output voltage of 3.7 or less.

UPDATE

I took a look at Digikey and they had nothing with 12 volt input, 3.7V out and 5.6A or greater.  so I was thinking it would be impossible to find an off teh shelf solution.  Then I search Amazon and this comes up:DC 24V to 12V CC CV Step Down Volts Regulator Constant Current/Voltage 8A Adjustable Buck Converter 7-40V To 1.2-35V LED Driver Charger Module with Charging Indicator Solar Power Supply DIY  

and

DC-DC Step-down Constant Current &amp; Voltage Converter 4-38V to 1.25-36V 12V/24V Buck Voltage Regulator 5A 75W High Power LED Constant Current Driver Module for Lithium Battery Electromobile Charging