I am an Indian national holding a valid multiple entry French Schengen visa for four years. I would like to visit Athens, Greece to attend a conference in September. Can I travel with this French Schengen visa or do I need to take a fresh Schengen visa from Greece?
If you already went to France, you can use the French visa without any issue, provided you fulfill all the usual conditions (have health insurance, sufficient financial means, didn't stay too long in the Schengen area, etc.)
If you haven't been to France yet, it's still possible but you might need to explain why you got a French visa to go to Greece, see Can I use my Schengen visa for a completely different purpose and entry point? and Use of unused Schengen visa to travel to Switzerland
Furthermore, having a four-year multiple-entry visa suggests you are a trusted traveller (UK resident?) so I wouldn't expect any problem.