At $550, honestly, I would probably buy it -IF- I could get out the door at that price... meaning it's a private sale or if you could talk the shop down enough to eat the tax.
There are a few facts that are worth considering when looking at a Model 28-2...
They made MANY of these revolvers and many, many more of the dash-2 variant than any of the other variants. Also, there's very few revolvers that Smith & Wesson ever made to be specifically a budget, spartan, dull, no-frills model. That's not to take away from what the Model 28 is -- but that is exactly what it is. All the quality, durability and ruggedness and very specifically without the attractive finish and pretty visual features.
I think it pays to keep that solidly in mind when shopping for one. It's not that these aren't in demand or that these aren't worth chasing. But honestly, these guns will have a tough time being "collector" guns unless they have a rare feature that wasn't common to the model, and are in 100% condition with all original box & tools.
If getting a very good deal is high on the list, the Model 28-2 is one that you can snag a great deal on if you're patient. I got my 6-inch in early August 2013 from a LARGE local gun shop that is known as a good place but not as a place where you walk out the door with ridiculous steals on gun buys. Mine was missing the original wood stocks, it had a slightly altered thumb-piece on the hammer and it had a small anomaly in the bluing that was about .22cal in size, clearly visible. The lockup, timing, end shake and every single little thing that I look for in a used revolver were perfect, and I got out of the store, tax included, for $460. I felt like a champion with that buy and I still feel great about it just over a year later.
Yours, being a 4-inch and in better visual condition (I certainly expect) would be a decent deal at $550, but I'd want them to eat the tax. And if I was really excited about it... then yeah, I'd pay $550+tax.