Rebecca bought 3 * 6 = <<3*6=18>>18 bottles of soda.
A week is 7 days, so she will drink 4 * 7 = <<4*7=28>>28 half bottles of soda in four weeks.
A bottle is two half bottles, so she will drink 28 / 2 = <<28/2=14>>14 bottles of soda in four weeks.
Thus, after 4 weeks, she will have 18 - 14 = <<18-14=4>>4 bottles of soda left.
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