McSpike - 7:18 pm on Jul 29, 2010 (gmt 0)
Links value is based on the value of the page value the link is on.
In short: you can't pinpoint an exact scenario or patent because all 4 mentioned make prefect sense and can very well be (or are) a part of equation that determines the value of a linking page and a power a link passes over to the linked page/site.
Also, #2 falls under #4, so basically there is no #2.
You can't really pinpoint a certain ranking factor, because it's a mix of them that counts more than a single one.