Sounds like they need to each have a footing below the floor level. Then patch the floor around it afterward. So temporary supports are used (carefully) as they are repaired. Not sure if each would be done "one at a time" or not.

I'm curious if this is a newer home or if these are repairs to an older home.

Could a new home builder have missed four pad footings?

Without knowing the load the posts are carrying and the underlying soil, it's impossible to know how big or deep the new supports should be. Best to get an engineer's opinion.

If not, one at a time I'd go at least 16" into undisturbed soil and 40" by 40" with 1/2" rebar going both ways in the pad.

Worthy,

This house is easily 100 years old - an I am told it is part of another home, and was moved to it's current site in the 1960's. The basement foundation, under main part of house, is 22' x 22' and until the previous owner placed jack posts under the center of house (under staircases), there was nothing.

Ground is clay / dirt.

Purpose is to fix sagging floors in center of house...

That changes things!

Anyone who attempted to level a floor by placing jacks on non-supporting concrete may also have misread what was necessary to level the floor in the first place.

In other words, putting the posts in correctly may not solve the problem. You really should have the house looked at by a local expert--engineer, architectural technologist, framing carpenter--before attempting any fix.

These "This Old House" segments, part 2 and Part 3 cover the topic in some detail.

The fact that the posts have sunk two inches shows they carry a substantial load and are likely at the right place, but not properly supported. But best to get an on-site opinion that can take all factors into consideration.

Best of luck! And don't just let it go.