To cite depmixS4 in publications use:

  Ingmar Visser, Maarten Speekenbrink (2010). depmixS4: An R Package
  for Hidden Markov Models. Journal of Statistical Software, 36(7),
  1-21. URL https://www.jstatsoft.org/v36/i07/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{depmixS4}: An {R} Package for Hidden Markov Models},
    author = {Ingmar Visser and Maarten Speekenbrink},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {36},
    number = {7},
    pages = {1--21},
    url = {https://www.jstatsoft.org/v36/i07/},
  }