How to change the starting reference number in Natbib in a given bibunit?

Question

I'm using bibunit and Natbib packages with a numerical numbering.

\begin{bibunit}[plainnat]
    \nocite{ref1,ref2}
    \putbib[mybiblio]
\end{bibunit}

\begin{bibunit}[plainnat]
    \nocite{ref3,ref4}
    \putbib[mybiblio]
 \end{bibunit}

How can I set the starting number for ref3 and ref4 to be different from 1 (say 15), without affecting the starting number for ref1 and ref 2? The desired result is the following:

• [1] ref 1

• [2] ref 2

• [15] ref3

• [16] ref4

Answer 1

My answer to How to set the number of the first citation? can also be applied if one uses bibunits:

\documentclass{article}

\usepackage[numbers]{natbib}
\usepackage{bibunits}

\usepackage{etoolbox}

\makeatletter
\newcommand*{\newbibstartnumber}[1]{%
  \apptocmd{\thebibliography}{%
    \global\c@NAT@ctr #1\relax
    \addtocounter{NAT@ctr}{-1}%
  }{}{}%
}
\makeatother

\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@misc{A01,
  author = {Author, A.},
  year = {2001},
  title = {Alpha},
}
@misc{B02,
  author = {Buthor, B.},
  year = {2002},
  title = {Bravo},
}
@misc{C03,
  author = {Cuthor, C.},
  year = {2003},
  title = {Charlie},
}
@misc{D04,
  author = {Duthor, D.},
  year = {2004},
  title = {Delta},
}
\end{filecontents}

\usepackage{lipsum}

\begin{document}

\begin{bibunit}[plainnat]
    \nocite{A01,B02}
    \putbib[\jobname]
\end{bibunit}

\newbibstartnumber{15}

\begin{bibunit}[plainnat]
    \nocite{C03,D04}
    \putbib[\jobname]
 \end{bibunit}

\end{document}