First off, let us say this, YOU ARE NOT YOUR FTP!
For many people, FTP based targets and an FTP tests based off of RG were close enough to see great improvements in fitness. Just like the power meter replaced HR as the gold standard for training targets, now 4DP™ is replacing FTP as the new standard for training targets. Any time intensity goes north of FTP, the variation from rider to rider becomes quite extreme, so the “appropriate” targets for shorter high intensity efforts can be quite different for two people with the same 20min max power.
The longer you have been training with FTP based targets, the more adapted your body is to those specific intensity targets. That means, it is easier for your body to hit a fitness plateau. With 4 different metrics to base interval targets off of, you can now ensure every workout you do is perfectly tailored to your abilities. Even if you are someone who falls exactly onto the “average” scale that the 20min power test and FTP based workouts assume, then with FF and 4DP™, none of your workouts would change which means you have nothing to lose! At the very minimum you will get an insight into your Rider Type and Rider Weakness, something that RG and FTP only based training cannot tell you.