Since bootstrap 4 use div class="form-row" in combination with div class="form-group col-X". X is the width you need. You will get nice inline columns. See fiddle.
<form class="form-horizontal" name="FORMNAME" method="post" action="ACTION" enctype="multipart/form-data">
<div class="form-group">
<label class="control-label col-sm-2" for="naam">Naam: *</label>
<div class="col-sm-10">
<input type="text" require class="form-control" id="naam" name="Naam" placeholder="Uw naam" value="{--NAAM--}" >
<div id="naamx" class="form-error form-hidden">Wat is uw naam?</div>
</div>
</div>
<div class="form-row">
<div class="form-group col-5">
<label class="control-label col-sm-4" for="telefoon">Telefoon: *</label>
<div class="col-sm-12">
<input type="tel" require class="form-control" id="telefoon" name="Telefoon" placeholder="Telefoon nummer" value="{--TELEFOON--}" >
<div id="telefoonx" class="form-error form-hidden">Wat is uw telefoonnummer?</div>
</div>
</div>
<div class="form-group col-5">
<label class="control-label col-sm-4" for="email">E-mail: </label>
<div class="col-sm-12">
<input type="email" require class="form-control" id="email" name="E-mail" placeholder="E-mail adres" value="{--E-MAIL--}" >
<div id="emailx" class="form-error form-hidden">Wat is uw e-mail adres?</div>
</div>
</div>
</div>
<div class="form-group">
<label class="control-label col-sm-2" for="titel">Titel: *</label>
<div class="col-sm-10">
<input type="text" require class="form-control" id="titel" name="Titel" placeholder="Titel van uw vraag of aanbod" value="{--TITEL--}" >
<div id="titelx" class="form-error form-hidden">Wat is de titel van uw vraag of aanbod?</div>
</div>
</div>
<from>