Charging a power supply with a power supply without making things blow up?

[salil]

I have a bluetooth headset circuit (Samsung HM1100) that I'm trying to power off a 4v DC power supply instead of the included single cell lithium-ion battery.  The circuit has a micro USB connector which, when a 5v DC supply is provided, will charge the included single cell lithium-ion battery.

The problem is that when I unplug the battery and plug in the 4v DC power supply instead, the circuit appears to refuse to power up.  When I try plugging the battery back in, it still refuses to power up until I supply 5v DC to the micro USB charging port for a couple seconds so it can go into charging mode (according to the flashing LEDs).  It looks like when the battery is unplugged, something happens that prevents the circuit from powering up again until you apply 5v DC to the micro USB charging port.

So it would appear to me that there are 2 potential solutions:

1.) Figure out how to create an uninterruptible power switch so I can switch from the lithium-ion battery to the power supply while the circuit is on without allowing the circuit to power down
2.) Briefly supply 5v DC to the micro USB charging port while the 4v DC power supply is attached and hope nothing blows up.

Is there any way to prevent something bad from happening by doing #2?  I'd only need to do this occasionally in the case of a power failure and only for a couple seconds at a time.  This circuit is being used for triggering a transistor to activate a garage door opener.  Any ideas?