I designed mine to use 4 frames- I chose this number for several reasons: first in a horizontal situation the bees move through a vertical excluder- which means they pass through it and hit one face of one frame- and have to move beyond that to get to the next ones. In a normal vertical hive the bees can simply go up and will hit any of the upper frames. As no one has used a horizontal hive with flow frames yet it remains to be seen whether the bees will readily pass the excluder and work their way across all the flow frames. This is my biggest concern about my hive- as yet it is unproven whether it will work or not.
The next reason is economy; 4 frames are cheaper than 6!
Another reason is the overall length of the long hive- I wanted to keep mine manageable yet have a big enough brood space for a realtively strong/large colony. Mine ended up exactly the width of two 10 frame boxes- and can hold 14 brood frames and 4 flow frames. I think 14 brood frames will give me a chance to build up a pretty strong colony.
Another factor that influenced me was that I plan to leave the flow frames in the hive year round- I am hoping that just four won't create too big an area beyond the excluder for bees to keep warm- or for the cluster to move into leaving the queen and brood to get cold... If my theory holds then the flow frames can act as winter stores- and I reduce the work of having to drain and remove them at the end of each season- also they will be there if there is any nectar coming in in winter- which I think is possible where I live.
The final reason is my theory that in a backyard home hive situation- where time and labour are not major considerations- possibly 4 frames could be as good as six- provided they are harvested more regularly. Anhows- that's my thinking and why I chose just 4 frames...