you cant use open levers with closed callipers and vice versa but pretty sure you can use new open levers with older open callipers
The only difference between open and closed Pro or Sport levers is the reservoir cap. The open system has a plain black cap with diaphragm with alloy ring underneath, the closed has a piston inside cap that can be moved up and down by alloy wheel on top to adjust for pad wear and if fluid gets hot. The main part of levers is the same and you can just change the caps, as I have done, to use with each system.
The closed system calipers have pistons that screw in on assembly with springs/spring washers to retract pistons when you release the brakes. The open system pistons I think work the same way as car calipers, the o-ring seal on piston rolls to allow piston to move in and out, pulling the piston and pads back off disc.
I phoned Hope recently to ask if original M4 calipers have to be used with the Mini lever, hoping I could use my Pro lever with open cap. Was told that they would work but there'd be a lot of travel on lever. Built and bled system with caliper and open Pro lever, it works perfectly OK with just the normal movement of lever. If Hope are correct I suspect the caliper I have may not be an original M4 but an Enduro4 which can be used with open Pro or Sport levers. The original M4 and Enduro4 which came out before the M4 are near identical, I think, in appearance.
Please someone correct me if I am wrong.