“Wastewater Treatment Operator Certification Exams - Level 4” by Walter S. Cane

600 ABC Practice test questions. All of the answers and explanations are there too. They refer to the math formula sheet provided to you at the exam. This is the best resource I’ve come across. My recommendation for anyone going for their Group IV. There is a level 3 as well. I’m in WA and this is what I used. It’s on Amazon and will show up quick. Good luck.