As per topic, I want to gather my friends and play as oldchool days, but since the Diablo's interface is also old school I do not know how to do this.

any ideas to share with noob? please do.

There's nothing special to do different for 4 people that you wouldn't do for just 2 people. Have one person create a game on battle.net. Tell each of the other players the game's name and password. Have each of them join the game. If all players are set up to play multiplayer, it will all work. If any are not, those players will fail. If anyone fails, fix them to be able to play multiplayer in a two person game, then retry.

https://github.com/diasurgical/devilutionX/releases has a slightly more modern way to connect to finds over TCP or UDP. As a minimum the player hosting a TCP game will need to port forward 6112 so that the other players can connect. Also the other players will need to know his IP.