\documentclass[]{article}
\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{tikz}
\fancyhead{%
\begin{tikzpicture}[remember picture, overlay, outer sep=0pt, inner sep=0pt]
\fill[blue] (current page.north west) rectangle ++(\paperwidth,-4cm);
\node[green,font=\large\sffamily\bfseries] at (-14,5) {some text};
\end{tikzpicture}
}
\fancypagestyle{plain}[fancy]{}
\renewcommand{\headrulewidth}{0pt}
\setlength{\headheight}{4cm}
\begin{document}
fff
\end{document}