Can I use it in a 3 way / 4way circuit with existing switches? Can it replace either 3 way switch?

Hello Rebert, thank you for your inquiry! Yes, the DVCL-153P can be installed in both a 3-way or 4-way with your existing switches. In a 4-way application, the dimmer is only to be installed in the 3-way location. Here is a link to the install guide: https://assets.lutron.com/a/documents/368-4493_page_5.pdf If you still have further questions or concerns, you can contact our Support Team at support@lutron.com or call 1-844-LUTRON1. We're here 24/7, you can call us any day or anytime.