Hi Jason,
According to your description, you have added your own domain to Office365, but when you accessed your team site with your own domain, there was a ‘404 error’ message. If I have misunderstood your mean, please feel free to let me know.
As David’s suggestion, I need some detailed information for troubleshooting. Could you please provide us with more detail?
In addition, here are some articles about Add custom domain to Office365 and create Public website address with custom domain for your reference:
For P Plan:
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/ff637561.aspx
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/gg549209.aspx
For E plan:
http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637620.aspx#bkmk_troubleshoot?CompanyType=CompanyTenant&DapEnabled=0&HasLiteSKU=0&HasAdminPermissions=1
http://onlinehelp.microsoft.com/en-us/office365-enterprises/gg570301.aspx
@David, thanks for your effort.
Thanks,
Eben Liu