Center cells in longtable - all together

Question

I would like to find an easy way to adjust cells in my longtable to centered, while still using the p{} definition of the columns.

The code I'm using is:

\documentclass[a4paper]{article}
\usepackage{lipsum}
\usepackage{longtable}
\usepackage{colortbl}
\usepackage{xcolor}

\newlength\mylength

\begin{document}

\lipsum

\setlength\mylength{\dimexpr\textwidth-5\arrayrulewidth-8\tabcolsep}
\begin{longtable}{|p{.25\mylength}|p{.25\mylength}|p{.25\mylength}|p{.25\mylength}|}
\rowcolor{white}\caption{Test table\label{tab:test1}}
                \hline
                \rowcolor{blue}\textcolor[rgb]{1,1,1}{Term 1} & \textcolor[rgb]{1,1,1}{Term 2} & \textcolor[rgb]{1,1,1}{Term 3} & \textcolor[rgb]{1,1,1}{Term 4}\\
                \hline
                Test 1 & Test 2 & Test 3 & Test 4\\
                \hline
\end{longtable}

\end{document}

Which gives the following output:

How do I center the cells of the table? I would like to be able to perform both options:

1. Center all the cells in the table
2. Center some cells individually, and keep the others as they are

Answer 1

For first question, you can define a new column type

\usepackage{array}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}

and use C instead of p.

\documentclass[a4paper]{article}
\usepackage{lipsum}
\usepackage{longtable,array}
\usepackage{colortbl}
\usepackage{xcolor}

\newlength\mylength
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}

\begin{document}

\lipsum

\setlength\mylength{\dimexpr\textwidth-5\arrayrulewidth-8\tabcolsep}
\begin{longtable}{|C{.25\mylength}|C{.25\mylength}|C{.25\mylength}|C{.25\mylength}|}
\rowcolor{white}\caption{Test table\label{tab:test1}}\\
                \hline
                \rowcolor{blue}\textcolor[rgb]{1,1,1}{Term 1} & \textcolor[rgb]{1,1,1}{Term 2} & \textcolor[rgb]{1,1,1}{Term 3} & \textcolor[rgb]{1,1,1}{Term 4}\\
                \hline
                Test 1 & Test 2 & Test 3 & Test 4\\
                \hline
\end{longtable}

\end{document}

For centering individual cells, you can use \centering inside the cell like below:

\documentclass[a4paper]{article}
\usepackage{lipsum}
\usepackage{longtable,array}
\usepackage{colortbl}
\usepackage{xcolor}

\newlength\mylength
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}

\begin{document}

\lipsum

\setlength\mylength{\dimexpr\textwidth-5\arrayrulewidth-8\tabcolsep}
\begin{longtable}{|p{.25\mylength}|C{.25\mylength}|C{.25\mylength}|p{.25\mylength}|}
\rowcolor{white}\caption{Test table\label{tab:test1}}\\
                \hline
                \rowcolor{blue}\centering\textcolor[rgb]{1,1,1}{Term 1} & \textcolor[rgb]{1,1,1}{Term 2} & \textcolor[rgb]{1,1,1}{Term 3} & \textcolor[rgb]{1,1,1}{Term 4}\\
                \hline
                Test 1 & Test 2 & Test 3 & \centering\arraybackslash Test 4\\
                \hline
\end{longtable}

\end{document}

Remember to add \arraybackslash if you are using the \centering in the last column.