This study involves adults with IgG4-related disease (IgG4-RD), a condition where the immune system attacks healthy tissues, causing inflammation in one or more organs. The study tests a drug called rilzabrutinib against a placebo (a pill with no active medicine) to see if it helps control the disease and is safe to use.

Participants will take rilzabrutinib tablets for 52 weeks. The study will measure how long it takes for symptoms to flare up and how well the disease is controlled without needing more medication.

• **Duration:** Up to 60 weeks, including screening and follow-up. There is an optional extended study for 108 weeks.
• **Visits:** 16 visits are required, with an optional 9 extra visits during the extended study.
• **Requirements:** Participants must have IgG4-RD confirmed by a committee and be willing to stop using certain medications during the study.

Participants must be over 18, have a stable disease, and be willing to undergo imaging tests like CT or MRI scans. They cannot join if they have had certain infections, liver disease, or a history of drug abuse. This study aims to improve understanding and treatment of IgG4-RD.