To cite package fourPNO in publications use:

Culpepper, S. A. (2015). Revisiting the 4-parameter item response model: Bayesian estimation and application, Psychometrika. 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 = {2015},
  }