Reading 4 bits without losing information

I have come across with a problem I cannot seem to solve.
I have a type of file "ASDF", and in their header I can get the necessary information to read them. The problem is that one of the "fields" is only 4 bits long.
So, lets say it's like this:
From bit 0 to 8 it's the index of the current node (I've read this already)
From 8 to 16 it's the index for the next node (Read this as well)
From bit 16 to 20 Length of the content (string,etc..)

So my problem is that if I try to read the "length" with a bytereader I will be losing 4 bits of information, or would be "4 bits off". Is there anyway to read only 4 bits? Any ideas would be appreciated, thanks :)

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