As mentioned earlier, you can use the grid system to layout your inputs and labels anyway that you want. The trick is to remember that you can use rows within your columns to break them into twelfths as well.
The example below is one possible way to accomplish your goal and will put the two text boxes near Label3 on the same line when the screen is small or larger.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" rel="stylesheet"/>
<!-- HTML5 shim and Respond.js for IE8 support of HTML5 elements and media queries -->
<!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
<!--[if lt IE 9]>
<script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
<script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
<![endif]-->
</head>
<body>
<div class="row">
<div class="col-xs-6 form-group">
<label>Label1</label>
<input class="form-control" type="text"/>
</div>
<div class="col-xs-6 form-group">
<label>Label2</label>
<input class="form-control" type="text"/>
</div>
<div class="col-xs-6">
<div class="row">
<label class="col-xs-12">Label3</label>
</div>
<div class="row">
<div class="col-xs-12 col-sm-6">
<input class="form-control" type="text"/>
</div>
<div class="col-xs-12 col-sm-6">
<input class="form-control" type="text"/>
</div>
</div>
</div>
<div class="col-xs-6 form-group">
<label>Label4</label>
<input class="form-control" type="text"/>
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/js/bootstrap.min.js"></script>
</body>
</html>