Hi Peter,
I hope you are staying warm. 
That's a good question.  I've been using a 5 cell battery for a long time for my stunters, both profile and full fuselage. 
I made the choice to go with 5 cells probably because of the size and weight of the battery pack at that time.  Personally I think that the higher cell count 5 or 6 gives a better response when governed.  4 cells would be fine for a sport flyer IMHO.  If you'll notice most if not all the top electric flyers are using 6 cells now. 
I made the choice of 5 cells based on the size (really weight) of the airplane I was comfortable flying.  Which was about 570sq/in and 50 to 52 oz.  Anything larger was tough for me to fly especially in the wind.  I have small hands and wrists and just couldn't really control a 650 sq/in 65 oz airplane when the wind blew more than 10 mph. 
I developed a spreadsheet for figuring out the components for an electric system based on airframe weight.  Then from those numbers, I knew what size battery I needed, then went shopping for one that fit those needs and also being as light as I can get.  As you know some batteries are a lot lighter than others.  It seems that a good 5 cell battery for my requirements was about 2400mah 25C battery.  I found that the Zippy Compact battery worked really well.  They were about 1/2 oz heavier than Thunder Power 5S 2700mah batteries but at 1/4 the cost.  They have lasted me several years of flying.  Alas when I went to buy some more late this year, it looks like Hobby King has discontinued that size. Bummer.
The motor size really has nothing to do with the number of battery cells.  The motor's KV does however.  As a general rule, the higher the number of cells, the lower the Kv for the motor. 
Say hi to all for me.