Try this in your .htaccess. I would put it above any Google SEO code, but below rewrite engine on.
If that doesn't work out, or optionally... Make your custom 404.html as:
(obviously change example.com to your domain name.)
ErrorDocument 404 /index.php
If that doesn't work out, or optionally... Make your custom 404.html as:
<html>
<head>
<meta http-equiv="refresh" content="3; url=http://example.com/">
<title>Page doesn't exist. Redirecting you to homepage...</title>
</head>
<body>
Page doesn't exist. Redirecting you to homepage...
<br>
</body>
</html>
Content is the # of seconds before it will redirect. Set to 0 to make it immediate.(obviously change example.com to your domain name.)