To cite the package 'fourPNO' in publications use:

  Culpepper, S.A. (2019) fourPNO: Bayesian 4 Parameter Item Response
  Model. URL https://cran.r-project.org/package=fourPNO.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {{fourPNO: Bayesian 4 Parameter Item Response Model}},
    author = {Steven Andrew Culpepper},
    year = {2019},
  }

  Culpepper, S. A. (2016). Revisiting the 4-parameter item response
  model: Bayesian estimation and application, Psychometrika, 81,
  1142-1163. DOI: 10.1007/s11336-015-9477-6.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Revisiting the 4-Parameter Item Response Model: Bayesian Estimation and Application},
    author = {Steven Andrew Culpepper},
    journal = {Psychometrika},
    year = {2016},
    volume = {81},
    number = {4},
    pages = {1142--1163},
    issn = {1860-0980},
    doi = {10.1007/s11336-015-9477-6},
    url = {http://dx.doi.org/10.1007/s11336-015-9477-6},
  }