Please note: This is somewhat out of the norm and is not covered by the ASTM F2170 standard. The following answer is based on information we have received from experts.
A 5-year-old existing 4-inch slab has 3-inches of new cement. The total thickness is 7-inches.
The test hole should be drilled at 40% depth of combined thickness (7-inches) unless there is a vapor barrier between the two slabs. If there is a vapor barrier then the test hole should be drilled at 40% depth of 3-inches.
Same information applies to precast slabs.