4. Xiao Ming's mother went to the store to buy soap, the unit prices of brand A soap and brand B soap are 6 yuan and 9 yuan respectively. If Xiao Ming's mother used all her money to buy brand A soap, she could buy 2 more bars than if she used all her money to buy brand B soap, and there would be no money left. So, how much money did Xiao Ming's mother bring?
【Analysis】Let the number of A brand be $x$ blocks, then the number of B brand is $x-2$ blocks; $6x=9(x-2)$, solving for $x$ gives $x=6$, so mom brought $6 \times 6=36$ yuan.