Hello Skancel,
I'm not sure why this happened, as I'm not very familiar with the rules of this game. 
Could your opponent have activated some option called "Format Best of 5"?
However i would ask you to refer to the,

Game instructions: 

https://www.playcurling.com/help/en/

The FAQ: 

https://www.playcurling.com/faq.html#topic=skins

Regards
Dav7d

Hello,

Old game rules are formatted to award a certain score per end to players whose opponent leaves the game. Typically it is 3 for the first end and 2 for each end after, but it is also impacted by number of stones in the rings if opponent leaves mid end, or sometimes by scores of past ends, e.g. if opponent scored 4 last end, first end score after leaving would be another 4. I don't know why it is, but my best guess would be FOD is trying to make the most fair result to players. Sometimes accidental disconnections happen, when the disconnecting player is winning. It's kind of like the legal system, where you have to try to make things fair for both sides.

In your particular case, it is obviously frustrating to have this result occur and extremely bad sportsmanship for your opponent to leave the game this way. However at the same time we must think realistically and about sportsmanship on your end. Being down 4 coming into the last end. You are not winning the game and you should be shaking hands without playing it.