Let me clarify....I've never had a failure with the knot at any strength of tippet, not just the heavy fluoro.
To your point regarding the use of four pound test tippet, I think that the test may be a bit moot to me because it doesn't reflect realistic conditions for the type of fishing that I do. For tippets that weak, I use knots other than loops.
Are we still talking about a saltwater knot here like in the thread that originated this subject, or have we moved on to a discussion encompassing all fishing situations? If we're still on the salt, then I would again have to say that four pound test is not realistic, although it might be more prone to showing differences in strength. But does that difference translate into higher pound test tippets?