I wouldn't regard that as out of theory. For example the game could continue 4 .. d6 5 g3 g6 6 Bg2 Bg7 7 0-0 0-0 which reaches a position akin to a reverse English. It's a tabiya position that can also arise from the players starting 1 Nf3 Nf6 2 g3 g6 3 Bg2 Bg7 4 0-0 0-0. I've had that exact position 18 times over 40 years and similar positions even more often. White has next to nothing objectively but I considered it a good line for trying to knock over weaker players.ThomasThorpe wrote: Take for example a sicilian. I've seen this played before at minor level:
e4 c5
Nf3 d6
Nc3 Nc6
d3
Ok, it's not the best example, but it proves that openings just 4 moves in at minor level can go out of theory,.
I'd say at higher levels starting at around 1900/2000, players will usually know (or can reproduce by general reasoning) the main lines of most openings and that knowledge of an opening is knowing the off-beat and new lines as well (or just knowing where the bits go and how the tactics work in the middle game).