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1.
A man who is single, and has expressed intentions of meeting a woman who has particular traits, can cite rule 4. This rule means his friends must greenlight an introduction between the two if said friends mention this desirable babe. see examples.
Friend:"So, Jim's girlfriend has this nice blond roommate and-"
single male seeking blond:"Rule 4."
Friend:"Shit... Talk to Jim about it, maybe you can meet her."

Friend:"So, this chick rear-ended me today-"
Dude:"OMG, I love Asians!!! Rule 4"
Friend:"Here's her insurance information."
by SingleGuy4956187597492 July 14, 2008
 
2.
No female apears to be unnatractive while performing felatio upon you.
John- "Dude that chick's ugly"
Tony- "Dude, rule 4: no girl looks bad with your dick in her mouth"
by fa-la-la-la-latio October 21, 2010