Aside from the interval analysis, you may want to do a fuller comparison with one of the tools shared here:
Then some basic stuff to cover:
- Is your 4iiii power meter properly calibrated (zero offset)?
- Is your 4iiii power meter a single-sided or dual-sided? If single, realize that is a notable variable when compared to the Kickr which is “Total Power” vs “Single-Sided Doubled”.
- Is your Kickr properly calibrated (spindown)? I think this should auto calibrate, but it may be necessary to take the extra steps in a case like this.
- Recognize the difference in locations measured via crank arm and post-hub (trainer). This introduces drivetrain efficiency into the variables.
I’d personally avoid using the calculators you used as there are likely variables at play (wind, rolling resistance, etc.) that make split comparisons rather flawed.
Your best bet is to do a set of data collection with the bike & power meter on the trainer (all proper calibration performed) and capture ride files for detailed comparison.