Use a micro power comparator, like the LT1716. Also, use a 2.5V micropower reference. Then, set up your input so it powers the comparator and reference, and a voltage divider. The divider should get to 2.5V when the input rail gets to 3.5V. The noninverting input goes to the reference through a resistor. There is a resistor of approximately 5 to 10 times the value from the noninverting input to the output (that's your hysteresis). The inverting input gets the divider. The output goes to your circuit you want to power.
As the input is rising, the divider will be below the reference (which will track the rail up to 2.5V). Once the rail gets to 4V, the divider will be above 2.5 + the hysteresis, and the comparator will start sinking current.
Note that you shouldn't pull more than about 40uA from the circuit, or it'll drag down the voltage across the 10k input resistor. Thus, make sure your reference isn't going to suck up alot of power, and make the divider very high resistance. When the circuit it on, it'll pull current from the input out to ground through the comparator.