The value is 4 and since it is in the context of "4 test tubes" the "4" indicates how many test tubes we have.
The number of significant figures are there to acount for uncertainty, so if we say that 4 has only one significant figure, it means that we don't know for sure it we have 4.3 or 4.4 or 4.0, etc.
However, since this 4 represents the number of test tubes, we can't have 4.3 test tubes, either we have 3, 4 or 5.
So, when we say we have 4 test tubes we know that we have exactly 4 of them. This is equivalent of saing we have 4.000000000..., because we know the value exactly.
So, the "4", in this case, have an infinite number of significant figures.