The Pololu AVR library, which can be used with the Arduino IDE, supports pin change interrupts. You can find more details about it under the "Orangutan Pulse/PWM Inputs" section in the Pololu AVR Library Command Reference guide.
You can use the Arduino IDE to program your Baby Orangutan, however, not all existing Arduino library code and other third party libraries are guaranteed to work with the Baby Orangutan. Please see the note under the "Arduino Libraries for the Orangutan and 3pi Robot" section of the Programming Orangutans and the 3pi Robot from the Arduino Environment guide for more details. You probably would need to test those libraries for yourself.
According to the Wiki page for the
EnableInterrupt library, which can be found under the "More Information" section on its GitHub page (that you linked), the library supports ATmega328P-based boards. Since the Baby Orangutan uses an AVR ATmega328P, you might be able to use that library with the Baby Orangutan. Again, you would need to test the library for yourself.
By the way, you mentioned wanting to control four brushed DC motors from the Baby Orangutan. The Baby Orangutan uses the TB6612FNG motor driver, which can only achieve independent control of two motors. If you tell me more about your motors (like their operating voltage and stall current), and how you plan to use them (e.g. how much load is on them and do you need independent control of each one?), I might be able to tell you if the Baby Orangutan is appropriate or recommend another driver.