Absolutely Nasty Sudoku Level 4

Top solvers can try their hand at the One-Sided Gordonian Rectangle; Extended Gordonian Rectangle; Gordonian Polygon; and Alternating Digits. (But they can take comfort in knowing that no more than one will occur in any single puzzle.)

• Absolutely Nasty Sudoku Level 3
• Absolutely Nasty Sudoku Level 2
• Absolutely Nasty Sudoku Level 1
• Beyond Black Belt Sudoku: If you have to ask, it's too hard for you. (Martial Arts Puzzles Series)
• The Nastiest Sudoku Book Ever
• World's Hardest Sudoku Book: 200 of the World's Toughest Sudoku Puzzles
• Third-Degree Black Belt Sudoku® (Martial Arts Puzzles Series)
• Will Shortz Presents Surrender to Sudoku: 200 Irresistibly Hard Puzzles
• Sudoku 3: 200 Puzzles: Hard to Extreme
• Will Shortz Presents Mind-Melting Sudoku: 200 Cranium-Crushing Puzzles