To cite package fourPNO in publications use:

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>.

Corresponding BibTeX entry:

  @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},
  }