Possible Clues:

• Simple rhyme scheme
• Quatrain rhyme scheme
• Four-line rhyme scheme
• Rhyme scheme of a villanelle's quatrain
• Quatrain form
• Simple quatrain form
• Rhyme scheme at the beginning of 'Can't Knock the Hustle'
• Straightforward rhyme scheme
• Quatrain scheme, maybe
• Rhyme scheme at the end of a villanelle