Hi.
First you need a polarity steering circuit which is nothing else than a bridge rectifier connected with the AC terminals to the telco pair.
The bridge (+) and (-) terminals are ready then to supply the 4 Volts to your circuit just by connecting a 3.9V / 1Watt zener across them. Any telco line must be capable to supply 20mA.
If you do not want to use the zener in the case most of the current flows trough it, you will have to connect a resistor instead, selecting its value to trim the voltage and current to needs. The resistor may fall in the neighborhood of 120 Ohms.
( If the resistor is disconnected while the gadget is in operation, it may get fried from higher voltage presence in the line, up to 50 V; plus the possibility of an incoming ring call with 90VAC )
The telco line will be seized in that operating conditions and unable to receive a call. After some time, the central office equipment will time-out the dial tone, place a 'hang up' voice message; or go mute; or whatever their policy is for a off-hook line. I do have a microcontroller that communicates data via telco powered by telco.
I also have a pocket transistor radio that is powered from telco; four or five different strobe lights operating on ringing; A 9V battery charger that works from telco power too. Of course most modern telephones are telco powered too, with bells, whistles, displays, lights and microcontrollers.
Check the application notes for that chip; it may have sample circuits.
By the way, I saw caller identification devices at the 99 cent store yesterday.
Miguel
it mean: becauseI do not know your meaning of "bec"
Miguel