I am trying to use the VLOOKUP function to auto-complete an order sheet with
product information.

I am using the following formula in the C4 cell:
=VLOOKUP(B4,'Product List'!$A$1:$D$100, 4, FALSE)

It is supposed to search the product number (in this case, "t25") and find
the cost of that product, located in column D.

I can't get this to work whatsoever, and just get a result of #N/A.

Some other examples of product numbers include:
t25
t26
t66
x90

....if that helps at all in diagnosing.