{:packet, 4} means, that you need 4 bytes denoting the lenght of the message and no newline after that (unless its part of the message).
In elixir syntax the message has to look like this:
<<4::integer-size(4)-unit(8), "asdf">>
# or
<<0, 0, 0, 4, 97, 115, 100, 102>>
