I thought I'd read 76 miles, but it certainly was the longest 4-track line in Britain when the Sharnbrook Tunnel line was double all the way.
But I think it's absurd to say that the line from Roade via Northampton to Rugby is part of a 4-track WCML. Basically, the four tracks need to be together. The Quarry line is acceptable as it's clearly intended to be a quadrupling of the Brighton Main Line and diverges (presumably) because of geographical and constructional factors. Similarly I'd include the MML north of Bedford, despite the divergence in places between the two pairs of tracks and the existence of Sharnbrook Tunnel on one pair. I'd also consider the Metropolitan Line between the junction just west of Kings Cross and Moorgate to have been quadruple track, although the City Widened Lines switch from the north side to the south side of the original line and there are stations on both lines. You apparently wouldn't accept that as quadruple track because both pairs have stations along the way, but I think it's much more obviously quadruple than the line that wanders off to Northampton.