The definitive guide to completing the Annual Return is
Governance and Accountability for Smaller Authorities in England 2017: a Practitioners’ Guide to Proper Practices to be applied in the preparation of statutory annual accounts and governance statements, March 2017.
This is available on line from the NALC website at:
https://www.nalc.gov.uk/library/publications/2405-governance-and-accountability-for-smaller-authorities-in-england-2017-sections-1-5/file
Paragraph 2.15 relating to Line 4 (Staff costs) says:
This cell shows all the costs incurred by the authority in relation to the employment of its staff. It includes employment expenses which are benefits (for example, mileage and travel expenses) but it does not include payments made in respect of office expenses reimbursed to employees or the costs of engaging agency staff or consultants (these expenses form part of the amount shown in Line 6). Where the authority makes deductions for PAYE and National Insurance, and pays employer’s contributions for NI and pensions, then staff costs should include payments to HM Revenue and Customs and any pension contributions.