For example if I have an Antminer S4, which has a 1.4kW PSU, this is consuming 5.8amps? Therefore I can only have 40/5.8 = 6 S4's on this RCD?
Almost. You shouldn't run a circuit above 80% continuous. So keep your 40 amp circuit at 32 amps or less. 32/5.8 = 5x S4s.