You forget to include things like USB Mass storage, I may not be watching 100mbps movies on my phone, but why would I wait 60% longer to copy something to it.
The phone memory is more than capable of matching the class 10 speeds so transfer between device memories are also faster. Besides at R160 why would I buy anything slower.
Hell where can you even find a class 4 32GB for that price.
Class 4 @
Prophecy is R225 + shipping
Wootware is R247 + shipping
Sybaritic is R206 + shipping
Takalot is R248 + shipping