What do you do to filter Amazon by 4.5 or higher stars? What do you change in the URL (or any other thing) to do this? (Any method that works is good.)
This is not possible.
You can only view reviews all of the same rating or all of them together. You can't have it group reviews with four stars (★★★★☆) and five stars (★★★★★) together.
On top of this, Amazon only allows you to rate based on whole stars. Therefore you cannot filter by half stars, like 3.5 (★★★♦☆), which are non-existent as an option when reviewing.