Beep patterns generator
The simplest circuit is a small microcontroller.  Generating a beep is as simple as putting out a square wave.  Many micros have PWM generators built in, which can automate this.  However, your application is so simple that explicit code can emit the beeps once you decide how long the on and off parts will be.

Even a PIC 10F200 can do this, since you only need 4 I/O lines.  The main loop waits for one of the input lines to be asserted, loads up the counters for numbers of beeps and beep duration accordingly, jumps to the code that does the beeping, waits for all lines to be unasserted, and does it all again.