Sorry, only just spotted your comment! The Important/Non-Urgent cell gets turned into Must Later or Should Later, i.e. 2 or 4 on your list. If Must Later, it’ll only be beaten by real emergencies (Must Todays), so unless your day is filled with emergencies you’ll get round to it.

If it’s just a Should Later, you’ll often not get to it. But if this is a skill you ought to work on *daily*, then it’s a Should Today each day, rather than Should Later, so will be higher up (number 3). In which case you’ll do it every day, unless the day is filled with Must Todays/Laters (which shouldn’t often be the case).