20/4 is way too extreme, and I'd never recommend it. In that case one would probably have better luck with fasting one or two days a week, but that would have to be under supervision of a professional. The reason 16/8 has become the "baseline" for IF is that it's enough time for 3 proper meals a day, and maybe even a snack in between if you're active. It's what works the best for most people, but of course outliers always exist. Some might be able to do more or less.

Myself and people I've helped all have better effect from a minimum of 30 min cardio every day, than prolonged cardio every other day or fewer. It might seem counterintuitive but the body is all about "use it or lose it" (and don't abuse it, I like to add) and it tries to optimize for the stresses it's commonly exposed to. So if you do a lot of cardio every other day combined with extreme IF but do cardio on rest days, it's probably not going to prioritize the breakdown of fat the same and might even "store" it for the tough work coming the next day.