Most of the CopyFX community prefers MetaTrader 4 to other platforms. The potential number of investors here is higher, but the competition is also higher.
On the other hand, MetaTrader 5 and R StocksTrader, while containing fewer potential subscribers, may allow your strategy to be discovered faster.
CopyFX does not currently support cross-platform copying, but you can still list your strategies on multiple platforms by creating separate CopyFX trader accounts for each platform.