The model 27 was a flagship piece, the direct descendant of the Registered Magnum. The model 28 Highway Patrolman was essentially the same gun, with a "lesser" finish, (and lower cost) intended for police use.
Maybe the guys on the production line spent a little more time fitting the model 27 internals, but if they did, S&W never claimed they did.
Essentially, the differences between the two are these,
Finish: Model 27 has the S&W high luster blue. 27s could also be had in nickel finish. Model 28s came in "satin blue" ONLY*.
Grips: 28s came only with the standard "magna" grips (the smaller ones), 27s had the larger target grips, or the magna grips, depending on customer preference.
Barrels: 28s were offered in only two lengths, 4" & 6". 27s came in a variety of lengths.
Sights: 28s had plain black (standard service) sights. 27s had red ramp front, and white outline rear sights.
The 27 topstrap is grooved, to reduce glare. The 28 has the same area "bead blasted" to a flat finish.
Hammer & trigger: The 27 came with the wide "Target" hammer & trigger, the 28 came with the standard "service" (narrower than target) hammer & trigger.
Some versions of the 27 came with the wood presentation case, 28s never did.
(* according to some sources, there were a very small number (perhaps 5) model 28s factory nickel finish made for test by the Miami PD. If you find a nickel plated model 28 today, odds are 99.9% it was done aftermarket.)
Personally, I prefer the 6" model 28. I like the look of the satin blue better than the high luster blue, and my favorite has the wide target hammer & trigger fitted to it. (up through the 70s, and early 80s I think, S&W sold target hammer & trigger sets in blister packs, which could be fitted by any authorized S&W repair station, such as a local gunsmith). Sadly, they stopped doing that, and stopped selling barrels and cylinders as well, some time ago.
Nice 4" you got there. Enjoy!