@mht2k3 ... the Littmann Cardiology III is less expensive than the Littmann Cardiology IV. The weight of the bell on the Cardio IV is lighter than the weight of the bell on the Cardio III.
I currently have a Littmann Cardio III and a Littmann Classic III: both work well for me. I think the Littmann stethoscopes are louder than the other brands of stethoscopes.
FWIW ... there is a classic saying in medicine:
it is what is "between the ears" that matters (experience versus brand of stethoscope).
In other words:
Listening for the sound of a graded heart murmur will require knowledge, practice and experience.
Whenever you learn that an animal has a heart murmur,
practice listening for the sound of the heart murmur in that animal (and then practice "grading" the heart murmur). Heart murmurs are graded on a scale of 1-6 (with 1 being the faintest heart murmur: it is graded as 1/6, and is commonly known as a "soft" heart murmur).
If no animals are available, you can do the same thing with any human friends who have been diagnosed with a heart murmur. Feel free to practice and fine-tune your clinical skills by "listening for and grading" heart murmurs. [By the way, a soft 1/6 can be difficult to hear - even for some cardiologists.]