Such a list is not likely to exist. Bloom time is dependent on several variable factors, weather and temperature being especially important. In Z4, however, there are so few rhododendrons hardy enough for the winter lows that a list may not be needed. If you choose varieties listed as Z4 hardy with mid season bloom designation, you'll probably get what you're looking for, at least in most years.

As an alternative, go to the nursery when they're blooming and purchase them then.
You can go for early bloomers, mid bloomers, or late bloomers and group accordingly.

Mike

Thanks guys. Mike, the problem is when they bloom at the nursery the following year they bloom at a different time in my garden. This means to me that they must have come from another state or zone to begin with and that's why they are performing differently in my garden.

akamainegrower - I now live in zone 5. Sorry, I should have changed that.

The selection is wider in Z5 than Z4, but there are still many variables that effect bloom time, including the exposure in your individual garden. Even though you want to plant them all in a single bed, some will be more influenced by temperature than others. Lists, even fairly comprehensive ones like the one on the Greer Gardens website, apply to Oregon or at least the West Coast and may not be applicable to your climate.

A sizable majority of Z5 rhododendrons are listed as mid season bloomers, but that is no guarantee they will bloom simultaneously every or even most years. If you plant only those indicated as mid season you will very probably get some overlap, but it should be close to what you're seeking. Another drawback, though, is color. Red and purple bloomers tend to be late, so effectively everything will be in the pink and white range with maybe some cream and pale yellow.