Well, I'm not sure exactly what you want to know. How close the 4DP FTP compares to Xert FTP? Note that, by definition, the 4DP provides something more/different than simply FTP, so I'm not completely convinced that a 1:1 comparison is appropriate. But, as a longtime SF user (including a few 4DPs) and having the free Xert on my FR920XT (which I use for every SF workout), here's my experience. Only a few SF workouts have sufficiently high max power requirements early enough to stabilize the Xert result on my free Garmin IQ field. Still, when I do one of those, my Xert FTP is a few (less than 5) percent lower than my 4DP FTP. This may be due, partially, to my weak sauce trainer (Tacx Vortex Smart), which is woefully slow on the transitions from recovery to interval. And the Xert Connect IQ seems to have changed recently from using a full workout for the estimate to a time-limited period (1st 60 min?). I've seen many SF workouts where, at the end, the Xert Connect IQ field says something like 'workout time exceeded' with no FTP. I don't blame them (for the free version), but it does make the comparison more tricky than otherwise.

EDIT to fix a couple of typos.

For XERT, I'd suggest listening to their podcasts (probably the first one would be good enough to let you know how they calculate FTP).

While you are not required to do FTP tests with XERT as the system will adjust your power signature on the fly (and degrade over time if you do nothing), you will still need "breakthroughs" during workouts/races if you want to see your signature go up. I get mine to go up usually when doing Zwift races, or during outdoor rides where there is less structure to the workout and I end up crushing a long climb. But you could still do a variation of FTP tests indoor, or hard workouts that are meant to push you over the "limit" (hence the breakthrough). Sometimes your breakthrough could be a 20min interval, sometimes a 15s sprint at the end of a long sub-FTP interval, the system will pick those up and adjust your signature accordingly.

I like XERT... their library of workouts is neat, and the smart workouts that adjust "on the fly" if you are not hitting targets are interesting (ie, extend recovery if your interval was harder than prescribed, etc.). I can't say if I'm in for a long term, but so far I like the system and while being a small team, it's updated regularly. So for now, XERT+Zwift is the right combo for me with XERT being the analytical and trainer control side of i and Zwift being the "carrot on the stick approach", but to each their own. After having used Trainerroad, Sufferfest (though not the app, just the video workouts of the past), TrainingPeaks, Sporttracks and Perfpro, I only have good things to say for each of them.

I've tried both and prefer Xert. They both try and express your fitness beyond FTP, but Xert can do it without formal testing and 4DP is only available through formal testing. In fact the 4DP protocol is the least enjoyable testing protocol that I've tried and due to it's complexity of the protocol, difficult to properly execute and have good data to compare.