It seems to be one of things for which there is no definite answer and we've exhausted the most obvious reasons. Sometimes there is no answer, it can be applied to just about anything. Why do some lightbulbs last longer than others, why do some electrical appliances fail, when an identical model can last for years.
Whilst you may think all lines are identical, there's the possibility that it could be on a different MSAN, or different line card. Sometimes a certain line card may not perform quite as efficiently as it could, but it is still within acceptable parameters. Line 4 could also be on a port which is affected by a crosstalker that doesn't come into contact with the other lines..
Crosstalk can rob adsl2 lines too. We dont really talk about it so much on ADSL when compared to VDSL because the speed losses are smaller (usually kbps rather than Mbps) and lack of decent monitoring.... but its still there. When BE* came to this exchange, no one else had really heard of them.. I was one of their first connections and had practically zero crosstalk for quite a while . When I visited my exchange the MSAN only had about 2 dozen lines. That changed when O2 bought them out... and even more changes when Sky bought them out. What previously was an easy 24Mbps Annex_M dropped to more like 16Mbps by the time I moved away. A single disturber could literally just wipe 1Mbps off my sync speed at the flick of a switch. You can't always spot crosstalk on adsl graphs are there are fewer tones, but sometimes you could see the shallow bowl like dips on the bit load graphs. 250kbps FeXT is quite possible on ADSL2.