Regarding 404 error, may I ask what have you tried to troubleshoot your domain and settings at Cloudflare dashboard, as stated in article from below so far?
May I ask did you checked under your current/older web hosting provider for the existing DNS records which you have had? Maybe you have an option to export them and then you can easily import them at Cloudflare dashboard → DNS tab.
Otherwise, you can add the missing one and edit the ones which need to be.
At Cloudflare Dashboard, you can navigate to the DNS tab and therefore you manage your DNS records from there, either adding new ones or editting the existing ones.
Here are useful tutorials for help about DNS records and changing the IP for the hostnames:
NOTE: You cannot change/assign the IP address from Cloudflare network for a proxied hostname (DNS record).