Uhm... They are
completely different.
The III has a very tight draw, too tight even for me at this point, and I'm strictly MTl (and yes, that 'l' is lower case for a reason). The III also has an itty-bitty deck with barely any room for wick ends to touch the deck. 20 watts is really pushing the limits of a III and it's really more comfortable at 10-15.
The IV has a much more open draw. Yes, you can close it down fairly tight, but IMO, it doesn't
vape well that way. This is an atty that wants considerably more power and airflow, more than I want to deal with.
For me, the III is too little and the IV is too much. I
used to love the III. Although I had high hopes for it, I do not love the IV.