Darkyyy, Welcome to ArgonathRPG IV:MP, I would like to tell you..
If you dont change the client and just use it, go ahead 
IV:MP is legal software and it uses Grand theft Auto IV as engine
So If you are going to use IV:MP as engine and you will create
extra program to lunch straight to ArgonathRPG server, then I think it is allowed but you still need to take permission from IV:MP dev.