|re: "I want to have the next blank column (Cell A3)"|
A1, A2 and and A3 are all in the same column. They are in different rows, so I'll assume that you mean "the next blank row".
You didn't provide any discount percetages for 3 or 4, so it's kind of hard for us to give you an exact answer.
Assuming 3 = 15% and 4 = 20%, try this forumla:
=A1*(1-CHOOSE(A2, 0.05, 0.1, 0.15, 0.2))
The CHOOSE function will choose one of the values from the list of values (0.05, 0.1, 0.15, 0.2) based on the value in A2 (1 - 4).
If A2 doesn't really contain 1 - 4, then a Nested IF will probably be required.
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