💡 Detailed Explanation
The correct sentence order is: 'この仕事は ときどき(2) 難しいですが(0) 面白いことも(3) あるので(1) 大変です。' (This work is sometimes difficult, but because there are also interesting aspects, it's intense). Grammatical breakdown: 'ときどき' (sometimes) modifies '難しいですが' (it is difficult, but). Then '面白いことも' (interesting things also) pairs with 'あるので' (because there is/are). The third position (★) is filled by '面白いことも' (Choice 4 in 1-based indexing, or Choice '面白いことも').