The -4 686,66,629 revolver versions had the non flash chrome hammers and triggers. Smith was phasing out blued revolvers at this time and the rumor is that they used up the excess inventory of plain case hardened hammers and triggers in the stainless revolvers.
The 629-3 and 29-5 classic (full lug) revolvers have a unique to them longer, rounded front rear sight. In the 629-4 and 29-6 revolvers Smith went to the current rear sight still being used today. If you use a current rear sight on a 629-3 or 29-5 Classic you will have a gap in front of the sight.
Also of note is that the non classic versions (non full lug) of the 629-3 and 29-5 were still using the old square front rear sight and these were not factory drilled and tapped for scope mounting.. In the 629-4 and 29-6 versions all barrel styles wore the new rounded front rear sight and all were drilled and tapped.