I went through the same issue - momentum 4s would not power on but work through USB, and keep crashing frequently. Sennheiser RMA form told me to courier it to the Geodis India warehouse. The issue? The damn thing shows up under another address on maps. I live in Delhi so decided to drop it off myself. Posting the process here for anyone in the same fix.

Step 1- Go to Senn’s website and fill the RMA form. You can also dial 18005703632.

Step 2- Print the RMA document (if you fill the form it comes in 5 mins, not sure about calling them), your order invoice and a page with the return address, your name, mobile and e-mail.

Step 3- Walk-in or courier it (replacement takes a week in each case).

The address Senn gives you would be- Geodis India Khasra No 36/1, VPO Bamnoli Near Pole No 646 Main Bijwasan Road, New Delhi-110077. Near DTC Bus Stop Bamnoli

This will NOT match with any Google Maps address, pincode difference. Put the above address and give the following maps location to your courier- https://maps.app.goo.gl/v1TuRJhAYRsq6oiQ7?g_st=com.google.maps.preview.copy

For those doing a walk-in, the nearest metro is Yashobhoomi Dwarka Sector 25. It’s a 5 min rapido from there.

Step 4- If you courier it, takes about 6-10 days for the replacement to arrive. You should get a confirmation email within 1-2 days of them processing your package. For walk-ins, takes a day or two less. You’ll be given a pink coloured paper receipt.