1) Make sure your friends have added you to their friend list:
1a) They have to open the "Friend List" tab (5th from left), then enter your in-game name at the bottom and click the "Add" button. Now your name should appear in that tab with a heart (or half heart).
2) You click on the "New Table" tab (2nd from left), then select "4" as the minimal number of players, then click the "Create Table" button.
3) Now your friends should go to the "Matching" tab (leftmost) and see your table on the right side in the "Friends" pane. They can press the "Join" button there (sometimes it needs to be pressed twice). It looks like this, with "Ingix" being replaced by your in-game name, of course.
3a) Once they have joined the game, they are just spectators by default. To become a player, they need to press the "Play" button on the lower left.
4) You need to wait until all 3 of youir friends have joined and become players. If somebody else joins your game as a player (like Ingix2 in the picture below), you can kick them, by klicking on the indicated place.
Most such players know that some tables are made for friends just as yours, but since they can't know beforehand, they join in in the hopes to get into a 4p game.
5) Once all 4 of your are joined as players, each one needs to press "Ready" to start the game.
Have fun playing!