I have data that is more than 3 or 4 years old. Can I email the concerned clients and ask them if they are happy to stay on our system and receive newsletters?

If they haven’t been to your salon in the last 3 or 4 years, chances are they’re just not that into you anymore. We advise against doing this. After 3 years, the argument of legitimate interest as a reason to contact them is no longer valid. Also, it probably won’t prove to be very efficient.