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Fifteen words carry Q second and U third, and the fourth box comes down to a straight choice.
A takes the fourth box in seven of the fifteen and I takes seven, an exact tie. Only three letters appear there in total, which makes this one of the most constrained boxes on the site.
SQUAB and SQUIB mirror them exactly, changing the same vowel. SQUAT, SQUAW and SQUIT follow the same shape again, so the A and I branches run almost in parallel.
That makes the fourth letter the whole puzzle. Confirm it and you are choosing between a handful of words that differ only in the last box.
EQUAL and EQUIP are both everyday words and both open with E rather than S. E takes four of the fifteen, with S on nine and A on two.
Every entry on this page follows the Q with a U, which is what English almost always does. The exceptions in the language are borrowings, and none of them lands in this pattern.
EQUAL tests the E and L and settles whether the word starts with S. SQUAD sits in the larger branch but tells you less about the front.
A squab is a young pigeon and a squib is a small firework or a short satirical piece. Only the fourth letter separates them.
Both are plurals of aqua, water in pharmacy and Latin. AQUAE is the Latin form and AQUAS the English one.
For an electronic oscillator to become unstable and produce bursts. It is engineering vocabulary and the least guessable entry here.