One of the nice new features in Skype 4 is being able to edit contact phone numbers. In previous versions, you were at the mercy of whatever someone had chosen to publish about themselves. If you had their number but it was not in their profile, you had to create a new SkypeOut contact. While you can still do that in Skype 4 (and get the benefit of this contact also working on other platforms), in Skype 4 you can also add a phone number to an existing profile.
It's straightforward: just click the big button that is aptly labeled "Add a telephone number". (For some reason, echo123 gets special treatment, I can't add any numbers to her. What if I really DO know her number? But I can at least add a number to echo-chinese, her Chinese stepsister, which is what I am using here as an example.)
Deleting a number is trickier, I could not initially figure out how to do it. Turns out that once you add a number to someone, there will be two buttons to the right of it. The left one is "edit" (bug: it appears to be missing a tooltip) and the right is "add another". When you click "edit", you get another dialog which has more buttons in it. The rightmost of those looks like a trashcan and is "delete".