The minimum chunk of memory processed by any single unpack pattern element is a byte. Hence, when you unpack a 32-bit element using repeated B4 elements, you get a value that represents the high 4-bits of each byte and the low 4-bits ar discarded. This may be made clearer by:

$b = pack 'I*', 0x12345678;
print join'|', unpack '(B4)*', $b;
0111|0101|0011|0001
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As you can see the (four!) values returned by unpack are the high 4-bits from each of the 4 bytes (in reverse order).

What I think you are after can be done like this:

$b = pack 'I*', 0x12345678;
print join'|', unpack '(A4)*', unpack 'B*', $b;
0111|1000|0101|0110|0011|0100|0001|0010
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