7.9.2 Clearance to land
An aircraft may be cleared to land when there is reasonable
assurance that the separation in 7.9.1, or prescribed in accord-
ance with 7.10 will exist when the aircraft crosses the runway
threshold, provided that a clearance to land shall not be issued
until a preceding landing aircraft has crossed the runway
threshold. To reduce the potential for misunderstanding, the
landing clearance shall include the designator of the landing
runway.
The above is from DOC4444...
So if you get the clearance when the preceeding is passing the threshold, or as number 4, is there a big difference? The ATCO will always keep an eye out for whats happening, and intervene if things get to tight.