What does "IPv4 addresses in the same AS" mean?

AS refers to Autonomous System which in rough terms means "block of IPs that share common routing" or in more general terms "are from the same allocation block".

You're getting this warning because the nameservers are all in the same block and if that single route goes offline for some reason, all your nameservers go down. It's generally best to spread these out geographically to minimize your exposure to localized events.

They're just looking out for you here. Typically you should have 3-4 different nameservers on different backbone providers in different regions so that no single failure, even at the provider level, can take them all down.