Given the definition of IF = NP/FTP, wouldn't it imply that your FTP was artificially low? I tend to record an higher IF on rides that involve repeated hard efforts with periods of recovery (e.g. Zwift races) where my NP is often higher than my steady state FTP. But 4 hours at at an NP equal to my FTP is never going to happen. I very much doubt pros can do that either, although they would get a lot closer - maybe 0.9 ish.

Just looking at some pro data, Paris-Roubaix was won last year with an NP = 341W over nearly 6 hours. I would expect Van Baarle's FTP is well north of 400W, so that would be at an IF = 0.85 at best. With a shorter 4 hour effort, maybe he could hit 0.9?