Note: I didn't read the Ch.5 yet. All I say below is based on the first four.
There's nothing wrong with the writing, any given tree in your forest, figuratively speaking, any scene is good and well written.
The forest as a whole, on the other hand, is lop-sided and weird-shaped. Your story. has plot problems. And the plot problems (I was careless enough to feel it on my own hide) are the nasty ones. You need to rewrite a hell of a lot to correct them, you need to throw out many of the good, complete scenes (because the story is an integral composition of its parts that should fit harmonically).
In short, (lo and behold) the 80% of my chapter (a big-ass chapter, I must add) went straight to trashcan. Including all the work on translating it and polishing it *in two languages*. Yes, this pain is very familliar to me.
And I'm afraid your story has the same plot problem that my one once had. Maybe I'm wrong. But, unlike Sunshine, I think it's too *slow*. If you plan to change the tonality, you should *show* it to the reader. Preferably, from the start.
My suggestions:
1. Add a side scene(s) with some innocents getting brutally killed, right at the start. I saw such inclusions in the Sailormoon manga and I think they work well (ex.: Rei walks the street and finds a charred corpse of some pedestrian burned alive). You talked about the contrast. Do not stretch it too far apart. Let the the bright and the dark lines run parallelly right from the start. If done properly, adds a nice touch of thriller too.
2. Try merging the chapters - here I agree with Sunshine. I feel this strategy did help my own story.
3. Squeeze the «water» out of the first 4 chapters. I know, it hurts like you are drowning kittens or something like this, but it is often a necessary evil.
Basket-case means: 1 : a [...]
Thanks