How can I permanently fix a sagging floor?
Frankly, in an older home like yours seems to be, I would not try to correct so small a sag.  But at the same time, so small a sag should be easy to fix.

Yes, the typical solution is just what you propose -- place one or two 6' jacks in the basement below the sag and span them with a 4x4, underlying 2 or more joists.  And yes, cracked plaster is a risk.  But if the current sag hasn't yet created a crack, it's likely that correcting your sag isn't going to move the wall far enough to cause cracking.  Worst case, a plaster crack is actually pretty easy to repair; it's just a bit messy (patching, sanding, and repainting a wall).

In my opinion, correcting such a small sag is unlikely to introduce other problems (add stress to plumbing, electrical, shift door jambs, introduce trim gaps, etc).  Consulting a structural engineer is definitely overkill.  Most home renovation contractors should have experience with this repair.

"Getting to the Bottom of Sagging Floors"

"Fixing a Saggy Floor"