@function trwlheading-selector-generator($selector, $prefix: null) {
    @return unquote(#{$prefix}#{$selector});
}

@function trwlheading-allselector-generator() {
    $selector: '';
    @for $lvl from 1 through 6 {
        $selector: $selector + 'h#{$lvl}, .h#{$lvl}' + ', ';
    }

    @return $selector + '%headings';
}

@function trwlheading-isheading($string) {
    @each $heading in ('h1', 'h2', 'h3', 'h5', 'h6') {
        @if $heading == $string {
            @return true;
        }
    }
    @return false;
}

@function trwlheading-selector-generator($selector, $prefix: null) {
    @return unquote(#{$prefix}#{$selector});
}

@function trwlheading-allselector-generator() {
    $selector: '';
    @for $lvl from 1 through 6 {
        $selector: $selector + 'h#{$lvl}, .h#{$lvl}' + ', ';
    }

    @return $selector + '%headings';
}

@function trwlheading-isheading($string) {
    @each $heading in ('h1', 'h2', 'h3', 'h4', 'h5', 'h6') {
        @if $heading == $string {
            @return true;
        }
    }
    @return false;
}

Fix typo into the is-heading function
Fix typo into the is-heading function

'h4' wasn't recognized