Hi (hola?) @sergiop!
I’ve seriously started japanese in february, discovered WK in may, I’m already level 13 (hoping on being 14 tomorrow afternoon ^^) and I can only talk for myself of course but here is my experience. I’ve started reading the よつばと! series on my own from tome one since one month ago (so I was level 9), but I’ve been also participating in the にゃんにゃん reading group for one month and a half (~ level 7?).
And to be completely honest, at the beginning with the にゃんやん reading group it was super difficult to me. I couldn’t get a single sentence (not only vocab but also grammar problems). So my first attempts were to take sentence per sentence, write it down, do some grammatical analysis to identify nouns, particles, verbs… Look in the dictionary a lot… So a single page was taking me around a couple of hours. At the end, if I was unable to make it on my own I was doing a lot of questions, and I gladly found a lot of people open to help and try to solve the riddles ^^.
And this said, I was only able to follow two pages per day.
About a month ago, I started to side projects: a few points of Tae Kim’s grammar guide per day and よつばと! in full speed. I actually took よつばと! really calmly: I was first reading it in french to know the story, and once finished I took the same tome in japanese and did it super fast without trying to understand everything nor making the same breakdown and detailed analysis as with にゃんやん. Why? First, fun. I hate getting stuck. Second, I was able to get new words from context because I knew the story. Third, I really think this is helping me reading faster (I see I’m much much faster with hiragana than a month ago, since now I can read a manga in a couple of hours ).
And today I jumped into tome 5 directly in japanese, and I’m super happy of getting a lot of it!! I’m at page ~18 and I went to the dictionary maybe 10 times?
Personally, I think the thing that made the biggest difference to me was getting used to “non text-book grammar points”. Like the absence of particles like は, が, を, だ in a lot of places and the abuse of other particles like よ, ね, な… And that you will not get from WK, kanji or vocabulary list.
So, I cannot really tell you that on level 4 yotsuba will be super easy. You will probably suffer :). But you’ll probably suffer it on level 10 too :P. But, the earlier you get into reading, the earlier you will get unstuck with these kind of things that you learn by reading. And I can tell you is that you’re not alone and people in this forum will answer any question that pops into your head.