I submitted a a short note, less than 10 page long, to a very prestious journal in mathematics. It has been about 4 months and I have not heard back. I wonder if that means the note made it pass through the initial assessment and is under review or it simply doesn't mean anything. The status of paper reads: reviewing. How does the reviewing process differ from lower tier journals to much more selective and reputable( whatever that means)journals?
The biggest difference between top level journals and others is the care taken in the reviewing process. Four months doesn't sound especially long.
Perhaps the journal publishes definitions of its terms such as "reviewing" which might give you an idea. But likely, it is being looked at by other mathematicians with knowledge and interest in your area. If the paper is straightforward it might get reviewed quickly, but be rejected for lack of impact. If it is not straightforward it will take longer to asses.
It is also possible that delays can occur if a reviewer decides after an initial look that he/she can't give the editor proper advice and passes it back with another reviewer then needing to be found. This could occur for many reasons, not all related to the paper itself. Some of the time delay is also likely due to the normal end/beginning of year academic scheduling with holidays occurring in most places.