Shor the Duck's SG Collection, a curated problemset split into difficulties of the best problems on Codebreaker set by the SG IO Community!
Criteria for 4 star:
Requires slightly more advanced techniques/data-structures, and may require some more thinking than 3 star.
(BSTA, Non-linear DS, more advanced application of prefix sums, basic DP, basic applications of DFS/BFS/Dijkstra)
Should be challenging for Codeforces pupils, and suitable for Codeforces Specialists.
Around NOI 2024 q1 (problemsetter) level