Try this...
<input type="text" class="form-control" name="name" placeholder="Name" required minlength="4" #name="ngModel" ngModel>
<div *ngIf="name.errors && (name.dirty || name.touched)">
<div [hidden]="!name.errors.required" class="alert alert-danger form-alert">
Please enter a name.
</div>
<div [hidden]="!name.errors.minlength" class="alert alert-danger form-alert">
Enter name greater than 4 characters.
</div>
</div>