http://www.ebay.com/itm/400603204991Input voltage: 4V-35VOutput voltage: 1.23V-30V
Switching frequency: 150KHz
Do you mean it is not even possible because the voltages 5v (input) and 4v (output) are too close together?
Voltage : Min 4.8, Typical 5.0, Max 5.2 VDC
Power select - select the power supply for GPRS shield(external power or 5v of arduino)
current: max 450 mA
Absolute Maximum Ratings:DC Current VCC and GND Pins...... 200.0 mA
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