[2]
1) (that)
2) (who)
3) (that) ← [the only] cap:[唯一の]帽子<限定>
4) (that) ← [All] the students cap:[全ての]生徒<限定>
[3]
1) (to) (whom)
2) (about) (which)
3) (to) (which)
4) (what) (he)
[4]
1) The child (I saw in the park isn't Kenta).
2) (The subject Yumi is interested in) is math.
3) The bag (was not what I was looking for).
参考にしてください。