Problem Array Formulas with VLOOKUP

I use Array Formulas in Column B from B2 to B8 to find the Name of item, the formulas is
{=VLOOKUP(A2:A8,$A$15:$D$17,2,0)}, the result is correct.
But when I use in column COST, it is wrong, the formulas is {=VLOOKUP(A2:A8,$A$15:$D$17,IF(C2:C8=1,3,4),0)}
temp.JPG[ATTACH=CONFIG]

Thanks.

Please upload a sample workbook, it makes fixing this quicker. Thanks

It's saying the attachment is invalid so I can't download it but from looking at what you're trying achieve you shouldn't need an array, just the following in D2 and drag down:

Formula:

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File attached - I just put the formula mentioned above and it works. Let me know if this solves your problem.

Thanks