Yes, You can customize the comment form with the Bootstrap 4.
You just need to do some edits with bootstrap classes and HTML.
$args = array(
'comment_field' => '<p class="comment-form-comment"><label for="comment">' . _x( 'Comment', 'noun' ) . '</label><textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea></p>'
'fields' => apply_filters( 'comment_form_default_fields', array(
'author' =>
'<p class="comment-form-author">' .
'<label for="author">' . __( 'Name', 'domainreference' ) . '</label> ' .
( $req ? '<span class="required">*</span>' : '' ) .
'<input id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) .
'" size="30"' . $aria_req . ' /></p>',
'email' =>
'<p class="comment-form-email"><label for="email">' . __( 'Email', 'domainreference' ) . '</label> ' .
( $req ? '<span class="required">*</span>' : '' ) .
'<input id="email" name="email" type="text" value="' . esc_attr( $commenter['comment_author_email'] ) .
'" size="30"' . $aria_req . ' /></p>',
'url' =>
'<p class="comment-form-url"><label for="url">' .
__( 'Website', 'domainreference' ) . '</label>' .
'<input id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) .
'" size="30" /></p>'
)
);
comment_form( $args );
Please use this. I think it can help.