You can get one of the 4 star servants below for free during the campaign!
If you don't have a main attacker servant, Heracles or Ibaraki Douji might be the best choice.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
Saint Quartz Farming Guide
How to build an Arts team
Demon (Demonic) Servants & Enemies
Best Craft Essence: Craft Essence Tier List
Zhuge Liang (Lord El-Melloi II, Waver)
Jeanne d’Arc (Alter)
Minamoto No Yorimitsu (Lancer, Summer 2017)
Piece of Year 2030 (A Fragment of 2030)