Assassin's Creed IV Black Flag PC Multiplayer launcher for Steam. Compile this and add it as a non-Steam game in your Steam library. It'll launch just like the SP game, but straight into the MP portion. For bonus speed, delete or rename the following 4 files under AC4\multi\videos: abstergo_logo.bik ac4bf_logo.bik TWIMTBP.bik ubi_logo.bik

Program.cs

using System.Diagnostics;

namespace ac4bfmp_launcher
{
    class Program
    {
        static void Main(string[] args)
        {
            var p = new ProcessStartInfo(@"path\to\Ubisoft Game Launcher\UbisoftGameLauncher.exe", "-upc_uplay_id 437 -upc_game_version 0 -upc_exe_path [path_to_exe_base64_encoded] -upc_working_directory [working_directory_base64_encoded] -uplay_steam_mode");
            // for example, my AC4BFMP.exe is located at D:\games\Steam\steamapps\common\Assassin's Creed IV Black Flag\AC4BFMP.exe,
            // so mine looks like: "-upc_uplay_id 437 -upc_game_version 0 -upc_exe_path RDpcZ2FtZXNcU3RlYW1cc3RlYW1hcHBzXGNvbW1vblxBc3Nhc3NpbidzIENyZWVkIElWIEJsYWNrIEZsYWdcQUM0QkZNUC5leGU= -upc_working_directory RDpcZ2FtZXNcU3RlYW1cc3RlYW1hcHBzXGNvbW1vblxBc3Nhc3NpbidzIENyZWVkIElWIEJsYWNrIEZsYWc= -uplay_steam_mode"
            //
            // note that you also need to create a file in your AC4 install directory called "steam_appid.txt" containing nothing but "242050" (without quotes)
            p.UseShellExecute = false;
            Process.Start(p);
        }
    }
}