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.
Yes, you can still access them. To see all of your Amazon reviews, you can go to this page (that works for amazon.com in the US). Each of your reviews should have a link to the product as well as pricing information if the product is still available.
I just looked through all the reviews that I have written, and some of the products did not show any pricing information. Those products are now discontinued and unavailable (no one is selling them - not even used ones), but I was still able to click on the link and go to the product page. For example, I wrote a review for the Viking CF256M 256 MB CompactFlash Card. Amazon still lets you see the product page, but if you do a search for that product, it will tell you that your search "did not match any products". So even if a product is no longer available, Amazon should still keep a product page for it, and you will still be able to see your review for it on both the product page and in your list of reviews.
In my list of reviews, there was also one review that did not have a link to the product for some reason. Instead of having a link with the name of the product, it just says "No Title Available" in bold black letters, and you cannot click on it. It looks like there can be some situations where Amazon deletes a product page, but I think it should be rare, because Amazon still lets people sell used ones (you can see in the product page I linked above, there is text asking "Have one to sell?" along with a "Sell on Amazon" button). But even if Amazon deletes a product page, it will still show your review for that product in your list of reviews.
Best Answer
Append the following to the end of the URL in order to have the items with the most reviews at the top:
&s=review-count-rank
It doesn't give exactly the behavior you are requesting, but it should make it easier to find products with a substantial number of reviews.