I have owned a Janome 10000 for over 10 years, and I love it, I have never tried the 4120 QDC. Overall, I think the Janome's are quiet and have less vibration then most of the others, but I think that you need to test drive the machine you buy, and also test drive the new machine in the store where you purchase it before you bring it home.
I want to welcome you to QB, you will find that all stages of quilters are open to share their ideas, suggestions and humor. Welcome to QB.