Four isn’t a bad rating, it’s a good rating. And while that’s just my opinion, as there isn’t any kind of international regulation when it comes to star ratings… I think it’s fair to say that it’s generally accepted that 1 is poor, 2 is below average, 3 is average, 4 is above average (or good) and 5 is excellent. More or less.
And forcing customers to do anything other than a basic sign up process and paying for content is never the best idea… it would just make buyers less likely to want to leave reviews. You can guarantee that your buyer isn’t going to leave a review again if he buys anything else… he’ll be too scared.
If they’d left you a one star review then I could somewhat understand your position. A 2 star review? Maybe. 3 stars? No. 4 stars? Absolutely not. While you’re entitled to your opinion, I really do think you’re completely wrong in the way you’re approaching this, and pestering a buyer for leaving you a four star review is absolutely ridiculous.