So to use trainer road, I use the matrix brand spin bike at my gym. It displays wattage on the screen (accurate within 2% they claim) but it does not broadcast wattage. So I just follow along and match the trainer road wattage on my phone to the wattage on the spin bike screen. To get my ftp I use the ftp ramp test built into the spin bike. The only problem i see is that the ramp test on the spin bike always starts at 75 watts no matter what. Will this be a problem, versus where a trainer road ramp starts you at a certain number based on your current ftp? Here’s the info on the matrix ramp test.
The second test that we use (and my personal favourite) is the FTP ramp test. The FTP ramp test is a progressive test and I love it because it allows people to learn about Power values as they test. The test structure begins by requesting just 75 Watts. The rider matches that with a combination of leg speed and resistance to suit them. The rider must then maintain that output for 4-minutes at which point the target wattage increases by 25 Watts. The target wattage will increase by a further 25 Watts every 4-minutes thereafter until the rider can longer maintain the target at which point the rider ends the test and gets their result. Everybody starts together but everybody finishes at different times. The test is easy to begin with but tough on the last 1 or 2 ramps for every rider as they’re likely above their Functional Threshold.
starting a ramp test at too low of a wattage is not a problem really, an extra few minutes at significantly below FTP is not going to build much fatigue. You could also just do a standard 20 minute test instead with the 5 minute blowout effort.
What about the fact it increases by 25 watts every 4 minutes versus TR increasing wattage every minute?
oops missed that part. longer(and shorter) step tests have been used by various researchers and organizations to determine maximum aerobic power(MAP) for example the Canadian and British cycling associations use IIRC a 3min step test. The conversion to FTP will possibly be different though and the 75% of MAP TR uses doesn’t necessarily correlate with true FTP since they fit their data to workout completion not 1 hour power correlation.
I’m guessing here, but the purposes of a FTP test to you in this situation are  measuring progress and thus incentivising yourself;  setting appropriate wattage levels for the TR workouts.
For the first, you could use any test you like, provided that you do the same test in the same conditions each time. Which it what it seems that you do.
But for the second purpose, you need a test that does what the TR ones – ramp, 8-minute or 20-minute – do. If the 4-minute step test that you describe leads to workouts that are hard but do-able [the harder ones leaving you feeling as though you could just do one more interval], then no worries. But if you need to keep adjusting the intensity of the TR workouts from that prescribed by the 4-minute FTP level, then it’s worth doing a more appropriate test, such as one of the TR ones. There is nothing to stop you doing one of those on the bike that you describe.