That's not an explicit "CL 4 required" thing.
That's because you're looking at the base rules, the exact CL is based on the Class. Like Wizard/Druid's minimal CL for a 2nd level Spell is three but a Sorcerer's is fourth. Then there are oddball Classes like a Paladin who only needs a CL of four for 2nd level Spells and fifth for 3rd level Spells. This is of course drawn from when they get access but it's more directly spelled out in the Magical Item rules.
Magic item rules are for magic items. Spells are not magic items. Magic item rules are not for spells.
There are official effects which reduce caster level -- it's easily possible to cast a spell at a level lower than the lowest at which you would ordinarily be able to create a magic item.
Spellwurm gives you an additional 2nd level spell known. That's an explicit problem. The Sorcerer doesn't simply have zero 2nd level Slots or Spells Known, he expressly has none and you cannot add +1 to a nonexistent value.
You're trying to nitpick, but you're not reading the actual text. Thus you're making stuff up and then trying to rules-lawyer based on your own inventions.
Spellwurm adds
mage armor,
blur, and
ancestral knowledge to your spells known. If you have a list of spells known (which you do, since you're casting spells spontaneously), these three spells are added to your "spells known" list. That's it.
There's no "additional 2nd level spell" in the text for you to build your case upon.
Often this sort of spell-list adder has some language about what happens if you don't already know spells of the appropriate level, but that language is absent here. If that language were present, you'd have a case. It's not, and you don't.
Colorful mock-up mockery notwithstanding.
Also VS does not allow you to count two 1st level Slots as a 2nd level Slot. It only allows you to use two 1st level Slots to cast a 2nd level Spell and that's kind of important too.[/size]
Quite true.
It's not a problem for this trick, since casting a 2nd level spell is the prerequisite that I'm targeting, but this part of your post is accurate.