I think that’s a fair observation, and as it is, I haven’t worked up to the full 4 x 8 minutes yet, and it’s been a while since my last test…
To complete the picture around the different FTP tests:
The 8 minute test is 2 x 8 minutes at 111.1% FTP (100/90) with 10 minutes of rest between. The IF for this is 0.99 for 26 minutes.
The 20 minute test is 1 x 20 minutes at 105.3% FTP (100/95) with a 110% 5 minute effort before that to reduce the anaerobic contribution. The IF for this is 1.01 for 30 minutes.
On reflection I could see the intensity being reduced back a little to get to the full 4 x 8 minutes.