7.4 Volts DC to 3-4 volts (small application)
Dropping voltage is the job of a buck regulator, linear (low-dropout or LDO) regulator, or various less-common methods such as a resistor divider, etc.

You cannot drop the voltage across a given load and maintain the same current through it. Voltage and current together define the quantity of power (watts) that is being delivered:

$$P = IE$$

From this (part of Ohm's Law), you can see that as you vary the voltage (E), you inversely must vary current (I) to maintain power (P).

A simple example: If you have 10 volts with a maximum current of 1 amp, you can use an ideal (non-existent) buck regulator to convert it to, say, 5 volts with a maximum current of 2 amps. In the real world, all conversions have losses, so you would more likely see 5 volts at perhaps 1.7 amps or so (85% efficient).

Your battery capacity is rated in milliamp-hours, which is a current-over-time sort of unit, specifying how long the battery can deliver a given current. There are plenty of other questions which discuss this. Again, a "perfect" battery with 1500 mAh rating could deliver 1.5 amps for 1 hour, or 3 amps for 0.5 hours, etc. Again, in the real world, batteries have internal resistance, and a myriad of nonlinear performance characteristics resulting from their varied chemistry.

Not only that, but battery datasheets will show you derating charts which give different mAh capacities based on the current they are supplying:



This means that a 1500 mAh battery might supply 50mA for ~30 hours, but get nowhere close to the 1 hour if supplying 1.5A.

It's critical (as comments have pointed out) that you know the requirements for the device you want to power at the lower voltage. You can then figure what sort of regulator will do the job (amount of power it can handle, efficiency requirement, and so on).

Hopefully this will help you out, and know what to search for. If you have additional questions, search first, then feel free to ask a new question (as a new post).