DOT 3/4 Brake fluid on 2003 Jaguar X-Type

Car was taken to shop and system bled. Told that the master cylınder may be bad because DOT 3 fluid was used in system. It states on the cap on resoviour that DOT 3 or 4 may be used. Have anyone heard of this? Can you suggest a good mechanic for tech assist or bleed assist.

Asked by for the 2003 Jaguar X-Type
What are the brakes doing now, How does the peddle feel because if the master cylinder is failing either the peddle slowly goes to the floor or it will leak brake fluid.
If you pump the peddle does it work better because if it pumps up you probably have air in the system, But if it slowly goes to the floor the master cylinder has failed. The master cylinder cost is about $550 and the labor to remove and replace is about 1 hour, Try a Napa autocare center
The pedal does go to the floor, but not by itself. The master cylinder is not leaking. Was working prior to replacement of rear driver side caliper and draining of fluid.