Have you restarted the server since updating your license key?
Does everything else (EG connecting via filemaker pro) work?
What's the admin console tell you? error log?
404 would indicate a complete failure of the WPE.
You may need to purchase additional web direct connections. Or deliberately turn on Web Direct.
FileMaker support is the best option. I assume your purchase entitles you to some days of free support and you would get better help there. Call and ask.
There may also be the issue of the latest update v9 not working well with servers that haven't been updated.
If your connection limit is reached, you get an error from FMS, not a 404. Good suggestion for a quick call to FM Support though if nothing you try works.