Hi Wiggy,
>>FWIW, in the UK, for precision approaches my operator uses 1000' agl as the control height ( no mention of 4 miles/ markers), and RVR deterioration or improvements after passing that point must be ignored.......( so if it drops below minima you do not have to go around.<<
My operators Ops Manual only refers to RVR deteriororations at/after the relevant point (usually 1000' AGL) It doesn't really make sense (practically or commercially) to ignore improvements after that point.
Cheers,
THICKO