From Sunday and Monday, Rachel drinks 2 + 4 = <<2+4=6>>6 glasses of water
From Tuesday to Friday, Rachel drinks 3 * 4 = <<3*4=12>>12 glasses of water
The total glasses of water she drinks from Sunday to Friday is 6 + 12 = <<6+12=18>>18 glasses of water
The total ounces of water she drinks from Sunday to Friday is 18 * 10 = <<18*10=180>>180 ounces
The ounces of water she should drink on Saturday is 220 - 180 = <<220-180=40>>40 ounces
The glasses of water she should drink on Saturday is 40 / 10 = <<40/10=4>>4 glasses of water
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