1.4 kW will pull about 5.8 amps at 240v so you are safe to put 5 maybe 6 on your circuit. How many outlets are you sharing on the circuit? To add more you would need to upgrade the panel to 200A then you could add 2 more 40A RCD's. Also I would spread the amps over multiple outlets because the wiring in one outlet is probably not sufficient to handle 40A.
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.
You should NOT give electrical advice assuming that the UK electrical circuits are similar with your US ones. Please, for heaven's sake .. grow up .. or the OP will need a new home soon. The only similar thing is that they have plugs .. see an example below:
