Why are you connecting to a multiprocessing listener with a raw socket?  You should be using multiprocessing.connection.Client to create a client connection.

Connection.send(obj) writes a 32 bit unsigned int (in network order) to the socket representing the length of the pickled data for obj, followed by the pickled data itself.

Since you are doing a raw socket write, the server connection is misenterpreting the first 4 bytes of your message "abcd" as the length of the message.  So the receiving end needs to allocate space for

    struct.unpack("!I", "abcd")[0] == 1633837924 ~ 1.5Gb

causing the MemoryError.