Disclaimer: not a Ruby user. I'm confused by example 4. Why would you return a message from either of them? Shouldn't messages in general belong to the view?
{{ user.address || "No address on file" }}
The "not so good" code is essentially this, but inside a wrapper in view code. What if you need different markup for a missing address, you either stuff it into a method or change the method's return value to nil... and what if only street_name is missing, not the whole address? It looks like a big mess to me.
Your example is not functionally equivalent as address is a model instance with many address-related properties. Though I agree with your general premise. This, to me, seems like the Tell, Don't Ask solution:
class User
delegate :street_name, to: :address, prefix: true, allow_nil: true
end
<%= user.address_street_name || "No street name on file" %>
