OK, 4-8 is tricky because it's neither one thing or the other. Go into bookshops & look at the banding, e.g Waterstones.
You'll see younger readers 0-3, 3-5.
Then 5-8, which is shorter chapter books, but does cover a pretty wide range. If this is where you're aiming it needs to be say, a "typical" 7 year old reader you're aiming at. Year 2-3 of primary. Boy or girl? At this age, rightly or wrongdifferent debate there are clear divisions in what appeals. Be clear in your head who your reader is.
Then 9-12, then teen & YA.
So you could just be falling within stalls on concept, word count & age banding. See if you can either rethink one of these - should it be a bit older, a bit younger? - or go back to the drawing board with a new idea.
Don't give up if you're committed to it.
And read, read, read. As much as possible in your chosen age banding. Note the patterns ... and then think about how you can break the mound.
Could be a talking animal book revival is due - or not. But you have to read loads to find out!
Sorry if this advice is sucking eggs...