M4 comprises: The M4 private sector's (i.e. the UK private sector other than monetary financial institutions (MFIs)) holdings of: sterling notes and coin; sterling deposits, including certificates of deposit; commercial paper, bonds, FRNs and other instruments of up to and including five years' original maturity issued by UK MFIs; claims on UK MFIs arising from repos (from December 1995); estimated holdings of sterling bank bills; and 35% of the sterling inter-MFI difference (added to OFC deposits, within wholesale M4). For further details please see the September 2011 Statistics article ‘Estimation and allocation methods within money and credit data’. Prior to September 2011, 95% of the domestic sterling interbank (now inter-MFI) difference was allocated to OFC deposits, the remaining 5% being allocated to transits. This followed a review of its causes (see page 101 of the June 1992 Economic Trends). The sectoral distribution of holdings of CDs cannot be fully identified; errors may affect M4 itself, as well as its sectoral analysis. Seasonally adjusted quarterly M4 is calculated as households' M4 (seasonally adjusted) plus PNFCs' M4 (seasonally adjusted) plus OFCs M4 (seasonally adjusted). More details are available in the Statistics article Seasonal adjustment of quarterly M4 excluding intermediate OFCs (M4ex) published in September 2010. Following a review of the seasonal adjustment method for non-intermediate OFCs once a sufficient run of monthly data was available, changes were made to the seasonally adjusted measures of M4 and M4 lending to bring them into line with the usual approach. The changes are detailed in the October 2013 Statistics article, ‘Modifications to the seasonally adjusted measures of M4 and M4 lending excluding intermediate OFCs’.

Source: https://www.bankofengland.co.uk/statistics/details/further-details-about-m4-data