Test 1 and Test 2 will have questions which check your knowledge of project management theory and PMBOK : these will help identify your weak areas for further study
Test 3 has situational questions and is very similar to final CAPM exam
Test 4 covers concepts (and has repetitions) from questions covered in Test 1, Test 2 and Test 3. It is very good practice after completing Test 1, Test 2 and Test 3 and should be taken before final CAPM Exams