I think four categories is too many, on the other hand I see the reason for separation.

You could have:
Precision Aerobatics (IMAC and Pattern), and
Freestyle/3D (IMAC freestyle and AA).

If the discussions revolved around flying techniques such as how to use rudder to keep your lines straight, trimming techniques, etc. then this breakdown would make sense. However the reality is that people end up talking about specific planes, like is the Extra or Edge better. Or are the PLProd or ZNLine pattern planes better. Fact is that the pattern guys don't want to see all those posts about the H9 Edge and the IMAC guys don't want to read about the Smaragd. However, both could benefit from discussions on trimming and using rudder. Therein is the snag.

Do we break the IMAC and Pattern forums to filter out all the stuff about planes that we don't care about and loose the benefit of crossover topics or do we keep them together and just skip the threads about planes that don't interest our particular bent?

I'm definilty for separating out the 3D stuff, and ideally I'd like to be able to read about only IMAC planes or only Pattern planes, but honestly I'm not sure I want to give-up possible crossover topics.

Finally, I definitely wouldn't split AA out as a separate topic because there will be virtually no posts in it. If we were to take YNOT's approach I would just do three categories:

Pattern / Artistic
IMAC
3D

My thoughts,
Keith