Make sure you are using the latest version there was a fix in a recent version
4.0.27
(09 Sep 2014) – Fix ‘After form additional HTML’ was not printing on form.
Fix to check for conflicting post types and show a warning. None of the field slugs can be the same as a post type rewrite_slug or you will get “page not found” when posting the form with that field filled in. In this new version, if you have this conflict there will be a warning on the form and on the form edit page. Warning: one of your field tags conflicts with the post type redirect tag “tag name”. To automatically correct this, click the “Save Changes” button on the form edit page. Also the form editor will not let a conflict happen while editing, but a conflict can still happen if you add custom post types. So if you add any custom post types, be sure to check your forms for any warning messages.
Oh yea I just remembered Some browsers disable meta refresh used for the redirect. So I still suggest disabling the redirect if it important to go to a step 3 2 type of a page.
That does not explain the 404 problem though
Use this solution 1:
You can turn off the redirect. You can edit the “Your message has been sent, thank you” text on the labels tab and add a link to the deposit page.
<h2><a href="https://www.url to your page">click here to continue</a></h2>
Also you can use the autoresponder to send them instructions about the deposit
This is a solution for some, but for the 404 problem you should skip this solution.
Solution 2:
OS safari does not support the meta refresh redirect feature. That is
stupid. But I have made you an easy work around.
Add this HTML to the setting “Success page additional HTML:” on
the Advanced tab
<h2><a href="https://www.url to your page">click here to continue</a></h2>
Now on iPad you do get the “Your message is sent” redirecting message
and there is a link “click here to continue” directly below it, the
link goes to the page you want.
All other browsers still redirect automatically