horizontal rule / line inside a multicolumn field

I need to have a horizontal line inside a multicolumn cell. When the horizontal line is just in one cell this is without problems possible, when spanning, with multicolumn, over say 2 cells I get an error message:

! You can't use \hrule' here except with leaders.
<argument> \hrule

l.22 \end{longtabu}


My MWE:

\documentclass[twoside]{book}

\usepackage{tabu}
\usepackage{longtable}

\begin{document}

\hline
Column 1&Column 2&Column 3 \\\cline{1-3}
Column 1&\hrule
&Column 3 \\\cline{1-3}
Column 1&Column 2&Column 3 \\\cline{1-3}
\end{longtabu}

\hline
Column 1&Column 2&Column 3 &Column4 \\\cline{1-4}
Column 1&\multicolumn{2}{l|}{\hrule
}&Column4 \\\cline{1-4}
Column 1&Column 2&Column 3 &Column4 \\\cline{1-4}
\end{longtabu}

\end{document}


The resulting image (going in batchmode on error):

In the second row of the second table I would like a line in the combined columns 2 and 3 like shown in the first table in row 2 column2.

I use the tabu package, I know that there are some problems with this package but I use the older version of Tex Live and for the 2019 version I use the "saved" tabu package (and friends).

Please use \hrulefill instead of \hrule

\documentclass[twoside]{book}

\usepackage{tabu}
\usepackage{longtable}

\begin{document}

\hline
Column 1&Column 2&Column 3 \\\cline{1-3}
Column 1&\hrule
&Column 3 \\\cline{1-3}
Column 1&Column 2&Column 3 \\\cline{1-3}
\end{longtabu}

\hline
Column 1&Column 2&Column 3 &Column4 \\\cline{1-4}
Column 1&\multicolumn{2}{c|}{\hrulefill} &Column4 \\\cline{1-4}
Column 1&Column 2&Column 3 &Column4 \\\cline{1-4}
\end{longtabu}

\end{document}


• My problem is nor getting ride of the message but that I cannot give a \hrule inside a \multicolumn and I need to have a horizontal line here. Just removing the \hrule leaves still the missing line problem. Note: inside the table \hline` doesn't work at all – albert Aug 29 '19 at 11:54
• @albert Refer the updated answer... – MadyYuvi Aug 29 '19 at 11:58
• Small understatement, this is much better. – albert Aug 29 '19 at 12:02
• I had to post a follow up question: interaction between \usepackage[table]{xcolor} and \hrulefill (tex.stackexchange.com/questions/506221/…) – albert Aug 29 '19 at 12:45
• Fine, its fine to go with a new question, but accept this answer is helpful for this question... – MadyYuvi Aug 29 '19 at 12:56