UPDATE This used to work (in 2007, I believe), but does not in Excel 2013.
This isn't quite the same, but if it's possible to put 0.4 in one cell (B1, say), and the text value A1 in another cell (C1, say), in cell D1, you can use =B1*INDIRECT(C1), which results in the calculation of 0.4 * A1's value.
So, if A1 = 10, you'd get 0.4*10 = 4
in cell D1. I'll update again if I can find a better 2013 solution, and sorry the Microsoft destroyed the original functionality of INDIRECT!
For a non-VBA solution, use the INDIRECT
formula. It takes a string as an argument and converts it to a cell reference.
For example, =0.4*INDIRECT("A1")
will return the value of 0.4 * the value that's in cell A1 of that worksheet.
If cell A1 was, say, 10, then =0.4*INDIRECT("A1")
would return 4.