Based on your shown samples, could you please try following. Please make sure you clear your browser cache before testing your URLs.
RewriteEngine ON
##Rule when www is present in URL so add only www to url.
Rewritecond %{HTTP_HOST} ^(www).hslab.nl
RewriteRule ^ http://%1%1.hslab.nl%{REQUEST_URI} [NE,R=301,L]
##Rule when www is NOT present in URL, hence add wwwwww to url as per OP
Rewritecond %{HTTP_HOST} !^www.hslab.nl
RewriteRule ^ http://wwwwww.hslab.nl%{REQUEST_URI} [NE,R=301,L]
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