Contao 4 en forum problem

You could also do the following:

[list=1][*]Under [i]System[/i] » [i]Settings[/i] go to [i]Security Settings[/i] and add [i]<script>[/i] to [b]Allowed HTML tags[/b] (if it's not there already).
[*]Go to [i]Themes[/i] » Your Theme » [i]Modules[/i] and create a new module of type [i]HTML[/i].
[*]Insert your Google Tag Manager Script there.
[*]Go to [i]Themes[/i] » Your Theme » [i]Page layouts[/i] and edit your page layout.
[*]Under [i]Front end modules[/i] add your newly created module to the list of [b]Included modules[/b].
[*]If you have multiple page layouts, you need to repeat steps 4 and 5.[/list]

[i]Note:[/i] best practice would be to create a custom section for such scripts in your page layout, preferably outside of the wrapper (may be only possible by default in Contao 4).