For making NModbus4 portable we should split up abstractions from transport implementation.
I'd suggest to split up on following packages:

1. NModbus4.dll - portable abstractions
2. NModbus4.TcpUdp.dll - portable Tcp/Udp transport implementation (depends NModbus4.dll)
3. NModbus4.Serial.dll - non-portable serial transport implementation (depends NModbus4.dll)

These changes will require some breaking changes e.g. we will have to delete some factory methods like:

public static ModbusIpMaster CreateIp(TcpClient tcpClient)
public static ModbusIpMaster CreateIp(UdpClient udpClient)
public static ModbusIpMaster CreateIp(SerialPort serialPort)

since they require arguments of types that not supported in portable subset. And introduce new factory for creating masters/slaves with corresponded transport.
For making TCP/UDP transport portable we also will have to reimplement it to rely directly on Socket instead of TcpListener, TcpClient, UdpClient, etc.

/cc @breyed if you are interesting in, any thoughts?