Since 1 foot is equal to 12 inches, then 4 feet is equal to 4 x 12 = <<4*12=48>>48 inches.
So, Marissa's sister is 48 + 3 = <<48+3=51>>51 inches tall.
The sunflower is 51 + 21 = <<51+21=72>>72 inches tall.
Therefore, the sunflower is 72/12 = <<72/12=6>>6 feet tall.
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