My feelings are that there's a BIG difference between "you don't know the main conflict" and "being kept in a cloud of 'what the hell is going on.'" The former makes for a cool and intriguing discovery when information is revealed slowly; the latter leans dangerously close to poor storytelling.
If a character's actions don't make sense to us, and we can't figure out their motivation, that all by itself won't make us curious, and you can't blow your readers' minds if they weren't curious in the first place. We have to CARE in order to be curious. If there's a Mysterious Flashback or a scene we can't understand, sure, we'll wait to find out what that was all about. But if more and more and more scenes don't make sense to us and we can't put ANYTHING together, we'll stop being curious and start being frustrated and uninterested.
We don't need answers right away, but we do need to gain more insight to our questions as we read.
The thing is, reading Four Quarters, right now you DO have a plot -- Nana was somehow transported to another world / dream world / otome game??? and she has to figure out what her new life is without letting on that she has no memory of said new life. So she has a goal, and obstacles, and her motivation so far ("I've escaped to a dream world somehow and I'm gonna milk it for all it's worth") makes sense. Regardless of how those goals, obstacles, and motivations will evolve in the future, your comic has a plot.
I think a tricky part is that for this kinda story, if you want to take your time building up to "the real plot" you GOTTA see this part of the story as a part of "the real plot" too. If you skip over and summarise conversations, don't really show us much of what the people around Nana expect her to be or how they click or don't click with each other, gloss over tense moments or potential conflicts, and keep their interactions kinda shallow, then we're gonna take that as a clue that this isn't the important part of the story, and that's when it'll feel like it hasn't got a "real plot".
When a story builds slowly, you don't want your foreshadowing to feel meaningless -- you want those moments to have meaningful emotion so that your readers remember them, even if they don't understand their full significance. Then you won't have a story that feels like it takes forever to get going -- you'll have one that ramps up even more when the true conflict is finally revealed.