I don't know anything about NetGally but felt I needed to give a differring opinion here. It's not for the sake of argument, but because I feel it's unreasonable for writers to feel they need to maintain a 4.5 or above rating if their book is any good.
The reason many promotion sites have a 4 star cut-off is to keep things sane. If promoters didn't put some cut-off they would get too many books in from 1 star on up, so a lot have stopped at 4 star. Sheer volume of books submitted is the reason, not readers being scared of less than 4 stars.
I have read over and over again that when a book is free or discounted it's the cover and blurb that get them to buy. I can vouch for it because it took me 2 YEARS to get a one star on Amazon. I think I had about 50 reviews, maybe 60. I don't remember for sure the total. Once I started doing promos with larger promo sites I got people downloading it who were not my target audience. That's fine, it's part of getting your book out there. This happens to everyone if they get enough people buying or downloading their books. And, of course, now that my rating has fallen one small site won't take me so I don't advertise my higher ratings with them either.
If the book was good enough for them before, well, it hasn't changed any.
Gastien now has over 160 reviews and a 3.4 rating. Truly, if any author wants to have a lot of readers they have to face the fact they most likely won't carry 4 star and above forever. Readers say over and over in polls they would rather see a book with a balanced amount of reviews than all 5 stars or all 4 and 5 because all good reviews make them suspicious. So obviously you can skew this rating thing and how it affects readers any way you want it.
All I know is I get more readers with a 3.4 than I ever did with a 4.5. The biggest promo site still takes my book. Probably because it always performs well. Every time it has ran it either goes over the average downloads for the genre or places in the high top. If my rating stopped readers those downloads wouldn't happen. And guess what? Along come more good reviews and more bad ones.
Unless you write something so bland it can't upset anyone you can hope for a 3 star average over the long haul if you get enough people reading it. Come to think of it, if it's that bland it WILL probably still get a lot of low rating.
The ones I see carrying four and above with hundred + reviews are usually romance. They are probably very well-written, but there IS a formula to follow which makes it less likely you're going to upset readers. NOt that romance is easy. I'm just saying if you know how to write romance and can write well you have the cushion of a formula to keep you within what readers expect.
If you write about characters or topics that make people feel great emotions all over the board, some will love you and others will hate you. Add graphic sex and some brutality and the same happens.
When a book is in a promo the readers don't read the reviews. If they did, they'd know mine has graphic sex and wouldn't have to read it and then complain it had graphic sex in it.
95% of my one stars are because of the sex. It isn't sex for excitement, it's about power, abuse of class, and trying to find something that momentarily takes away pain, but some readers simply read a sex scene and assume it's supposed to be erotic. Then again, some readers call it a romance in their review (both good and bad reviews) and it is not a romance, nor is it in the romance categories. Readers have all kinds of opinions and ways of picking books, and we can't completely control it. Am I complaining? No. I'm just saying, if you get a lot of downloads it happens. I knew writing this book I would end up somewhere between 3 and 3.5. I'm proving myself correct.