Step 1: Read each sentence for its real meaning.
Sentence 1 is a rhetorical question, "Shouldn't they have checked your tickets?", and such questions usually carry the speaker's own belief that yes, they should have been checked. Sentence 4 states directly, "They should have checked your tickets," which is the same belief stated plainly.
Step 2: Compare with the other two sentences.
Sentence 2 says "I wonder if they should have checked," which shows doubt or curiosity, not a firm belief, so it does not match sentences 1 or 4. Sentence 3 asks "if they checked," which is about whether the checking actually happened, a different question from whether it should have happened.
Step 3: Group the matching pair.
Since sentence 1 is a rhetorical way of asserting the same claim as sentence 4, these two convey the same idea, while 2 and 3 stand apart in meaning.
Final Answer:
Sentences 1 and 4 both assert that the tickets should have been checked, so option C is correct. \[ \boxed{\text{C}} \]