[4] Over the years, the X4 route has linked the main towns of the county of Northamptonshire with Milton Keynes and Peterborough enabling better east-west county public transport links than has previously been possible.

[6] Initially Stagecoach X7 ran between Northampton and Milton Keynes instead (as well as to and from Leicester), but this was also cut back leaving the X6 as the main link between the two towns.

The X4 inter-urban bus service benefited from £4.4 million ODPM Growth Area Funding between 2003 and 2006 and has seen annual passenger numbers rise from 325,000 in 2003/04 to over 500,000 in 2004/05.

[7] In November 2011, this route became a Stagecoach Gold branded one with Stagecoach Midlands spending £1.9 million on a fleet of 13 new Scania ADL Enviro400 buses based at the Kettering depot, halfway along the route.

[8][1] These buses have Lazzerini leather seats and free wireless internet [9][7] In October 2018 the fleet was replaced by 13 Alexander Dennis Enviro400 MMC buses by Alexander Denis at a cost of £3.1million [10]