Hi Jeff,
My dad always said, "the only stupid question is the one you don't ask." We deal with these types of situations on a daily basis. In my professional opinion I would recommend testing all ground floor units in the four buildings,especially if this is being requested after prior readings were elevated.
In the perfect world if you had copies of the prior reports, that would be helpful in determining what protocol was followed initially and what units were actually tested in all the buildings. Not having this information, one would have to consider this as initial testing. If this is not a HUD project, then the 100% plus 10% of the upper units in all the buildings would apply per the ANSI/AARST standards. If you are protecting your clients interest in this property and covering your assets :-) performing a full survey should be the way to go in order to determine the need for mitigation at the entire property. I do realize however that budgets are always an issue and you have to deal with that as well. If the testing is only performed on the four buildings and without the full knowledge of the prior results your testing would have to be considered a limited screening and not a full radon survey. This also can be the push your client may need to gain access to copies of the original testing reports as well. I hope this helps.

I agree with Tammy, It depends on what is known. When I read your post first time for some reason I was assuming you were the testing company that had done the first phase testing as a pre-purchase testing for that same client, thus that you had generated all the reports and did 100%/10%/10% already. In that case I discuss with client that the ANSI/AARST protocol only requires those that were high to be tested in Phase 2. If I do not have that report, or the report shows the first test was not following ANSI/AARST protocol, I propose to follow that protocol.with 100%/10%/10%, letting them know that 100% testing of all rooms on upper floors is not required by the protocol. However if they insist they do want 100% of all units per building I would give them an estimate with each Option A and Option B and let them sign which Option they choose.
Furthermore, it is important and I make a point that a single test per building in a pre-purchase test (not compliant with ANSI/AARST) showing a low radon test result does not exclude other units in that same building to be high and that a new Phase I test following the ANSI/AARST protocol could resolve that in the other buildings that were low (adding an Option C).

Thank you Tammy and Leo! Very sound advice from both of you and I greatly appreciate you both answering quickly.

Good morning Jeff,
I'm in agreement with Tammy and Leo. The key here is to get all possible information to make a proper decision regarding potential mitigation. So the 100 O of ground contact units and 10% of above ground units is definitely the best way to go. That would give you 16 tests locations in each building (12 +2 +2 always rounding up on each floor).

It is a shame that the original testing results are not available, but as Leo indicates, they may not be valid anyway. From a cost standpoint, this is much cheeper than treating a few units and then in clearance testing find out that the entire building should have been treated. (Although the entire building approach is usually the best.

Good luck with the project.