I want to run 4 devices that run on 3 to 3.3 volts (2 aaa, aa, c or d cell batteries). I am going to get a Lithium-ion battery pack that is 3.7 volts with 15.6 aH's. Yes this will make a very long run time since the devices don't use a lot of current. Here is the question, will 3.7 volts harm the devices that only use 3 volts? The battery pack when fully charge is slightly over 4 volts but this is the closest voltage I can find in a high capacity lithium-ion pack. This is all DC by the way as well. If it is too much what can I solder in line to reduce 3.7 volts to 3 volts that could handle 1 to 2 amps of current max without burning up? Thanks very much.

Do these devices have any distance to the run? if so, the voltage drop may do that for you?
Low voltage drops fast.

No, the devices and the entire circuit and setup are all within a square foot.

I want to run 4 devices that run on 3 to 3.3 volts (2 aaa, aa, c or d cell batteries). I am going to get a Lithium-ion battery pack that is 3.7 volts with 15.6 aH's. Yes this will make a very long run time since the devices don't use a lot of current. Here is the question, will 3.7 volts harm the devices that only use 3 volts? The battery pack when fully charge is slightly over 4 volts but this is the closest voltage I can find in a high capacity lithium-ion pack. This is all DC by the way as well. If it is too much what can I solder in line to reduce 3.7 volts to 3 volts that could handle 1 to 2 amps of current max without burning up? Thanks very much.

I would hook hup a meter, power up, check the voltage and power down, you may be closer than you think. A little momentary overvoltage should not hurt. Plus as the pack wears down, the voltage will drop.
You can always use a regulator.