Dot 4 is a good replacement (or equal to the expensive SMT-fluid). And why know one heard of the SMT-fluid is probably because the system just works with no faults on the majority of cars. There isn't even any maintenance of the fluid so it's probably supposed to just work the hole lifespan of the car as long as no leakage appear. Seems logical because there isn't any real wear of the oil. It doesn't get warm like in a brake system. It just get pressurised and as long as the reservoir tank isn't empty when the pressure has build up I don't think that any top up is necessary.