Looking at the questions, only Q2 has an answer that doesn't reference itself directly. For example Q1 asks about c) as an answer, but its answer c) is Q1.

So it's easiest to look at Q2 first.

Q2's answer can't be b) (as this would be an immediate contradiction), so Q4's answer can't be a), so Q2's answer can't be a).

Q2 can therefore be either c) or d).

If Q2's answer was d), then Q1's answer would be a), so Q3's answer would be c), so Q4's answer would be d), so Q1's answer would be b). However this now contradicts itself, so Q2's answer can't be d).

Therefore Q2's answer must be c). Making Q3's answer a), making Q1's answer d), confirming Q2's answer is c). Leaving Q4's answer as b).