Welcome Ric!
Lost of good info above.
As for rarity,, it's one of those questions that has different answers.
The "Old Models" which are the 3-screw action types built prior to 1973,, are one direction to consider. The earliest ones,, also known as models "RSS4" are among the harder ones to get. (There are sub-variations within even those.) Then there are the OM Lightweights,, which also have a 4-5/8" barrel. Those are also rarer to find to buy, and desirable to many. And you get into the later convertible OM's as well. (RSSM4X). And later on,, the Super Single-Six models SC4 came along.
So in just the OM's you can have a certain rarity factor,, especially due to the actual production numbers being lower than the more common 5-1/2" & the 6-1/2" were.
But you compared the Colorado Centennial model, which as noted above has a 6-1/2" barrel to a 4-5/8" model. Your info on that comparison is incorrect.
But after 1973,,, with the New Model design,, over the decades,, there have been different models & such that have been produced with a 4-5/8" barrel.
To many,, that barrel length is desirable,, and has occasionally been made in a variation of lower production numbers. As such,, they can be either "Kinda rare, to uncommon, to occasionally more common" depending upon the actual variation.
The best way to get good solid info on your gun is to spend $10 & get the factory letter. You can find a link to this on the Ruger factory site. the letter will give you the actual model designation, it's month & year of production, when it was shipped, and where it was shipped to. Ruger doesn't release production numbers,, but we collectors around here use a resource known as "The Red Eagle News Exchange" as a bible to our hobby. We can sometimes get a fairly good idea of actual production numbers because many of us report stuff to the publisher of the RENE. (Chad Hiddleson,, and he's known as "chet15" around here.)
But in general,, the 4-5/8" barrel length variations are most often produced in fewer numbers than other guns just like it.