Ok, a quick Larmo opinion,
soldier number one is slightly later although he wears an FP era shako, he is armed with the 1871 pattern rifle (general issue occuring around 1874) with the correct era cartridge box for that weapon. It also looks as if he has the 1871 pattern bayonet on his hip. A very nice image BTW Joe..
Soldier number two appears to have an 1860 pattern helmet (at least a cruciform base appears there to my eye), he has a black belt on which would indicate Fusilier Battalion, with a brass handled bayonet or faschinenmesser on his hip. I do not know if Fusilier battalions of regular infantry regiments were armed with the 1860 Fuslier Gewehr or only straight Fusilier regiments had them. Off hand I'd vote yes for FP era but I get in trouble with the litzen bit on his cuffs, he certainly has the look, long skirt to his waffenrock etc.
Soldier number three is 1870-1871 (or earlier) white belts for Grenadier battalions, socket bayonet for his needle gun and the hint of the brass handle to his faschinenmesser on his hip, he's great...this OYV
Soldier number four I would vote 1880's-1890's, It's possible he's earlier but he doesn't have the LOOK, how's that for a reasoned explanation

A nice clear image to be sure but later than the 1870 war.
IMO, in general German photography in general from that era tends to be rather grainy, without crisp detail (exceptions exist certainly) that you see in later photography.
Hope this helps....one man's opinion certainly
Sorry for the double post....HALP!!!