They did make the model 625 in longer barrel lengths up until some time in the late 90's or early 2000's. I think you could get 5", 4" and maybe even shorter lengths.
I believe the reason for them making the 4 inch barrel only models now is because in the US, IDPA (a popular type of action shooting there) changed their rules in about 2002, so that revolvers with a barrel longer than 4" were no longer allowed to be used. (I'm not 100% sure of this, it's just what I read on the Internet) Because of this rule change, the demand for the model 625 (which was popular with action shooters because it uses moon clips for quicker reloads) was limited to the 4" model. If they made a longer barreled version again, it wouldn't be nearly as popular in the US, so one can only presume it's an economic decision based on what they think will sell.
As an interesting note, the 625 4" model actually has a barrel longer than 4". I asked a US seller to measure it for me, because I was looking at seeing if i could get it imported. Here are the pics that he sent me (this is one of the 4" barrel ones) :
As you can see, the barrel is actually very very close to 4.125", which works out to 104.775 mm. In other words, if it was 1 mm longer, it would be restricted here, not prohib. In fact, there may be some manufacturing tolerance variations, and who knows, maybe there are a few out there with a barrel long enough to not be prohib here in canada, but I doubt you'll be able to find someone in the US willing to measure every one looking for the elusive (and possibly non-existant) one that's just over 105mm.
In the end, I bought a 5" one off Gunbroker and had it imported. More expensive (the 4" models are much more common, and consequently cheaper) but I was able to bring it in without worries about it being a hair too short.
This is the one I bought:
It's definitely a keeper.