It depends on what you are going to use it for. IIRC, oak and pine are a bit acidic themselves. Soil amendments tend to have some pH buffering qualities, but you might also want to add some lime.

The decomposed oak and pine bark will certainly improve the soil tilth, but the pH is a separate problem.

By the time it's compost, the pH is very likely near neutral.

If your soil's native pH is 4.2, I wonder if anything is growing there.

That said, most labs return the results along with suggested remedies. They should have indicated the amount of lime to mix into your soil on a regular basis. If not, call them and ask.

Beyond that, clay, in and of itself, isn't "really bad."

Soil sampling and test results are notoriously bad, I don't believe Ph 4.2.

That aside old bark piles can easily be very anaerobic and may be a problem simply moved and applied. Spread it out and be sure it has a chance to become aerobic then have at it. IÂve planted directly in 10 inches of material just like this on top of clay it did wonderfully.

I had cleared an area for construction down to bright red clay. I owned a compost facility and brought ground up hardwood and piled it 10 inches and sat 1 gal size root balls on top of the clay and walked away, it did require regular watering.

I'll trust a lab report on pH within a factor of 10. That's not as rough a measurement as it seems. Measurements of pH are logorithmic, if I understand it correctly. The important thing is that pH is not a percent. A ph of 6 is a LOT more acidic than a pH of 7, and 5 is a LOT more acidic than 6. And 4.2 would be bizarrely low.

So Jean001 and Esther Opal are correct; a pH of 4.2 would be very unusual and suspicious. You might ask the test lab to confirm the claimed pH measurement.

The pH scale is logrithmic, and each division is 10 times different than the previous on, ie. at pH 7.0 you have no free radical hydrogen ions running around in your soil, but at pH 6.0 there would be 100 times as many free radical hydrogen ions running around. Can a soil show pH 4.2? Absolutely, and you need to look at all they other parts of that soil test report to see why. Just a soil pH means little just as a report of Ca means little unless you see the relationship with Mg.
Can the addition of Oak and pine bark change the soil? Absolutely. It would take a very large quantity of this material to make a significant change in soil pH, although enough of it will buffer that soils pH so many plants can grow there without making a significant change in soil pH. However, bark would be a woody material and adding woody materials to soil, according to research done by Dr. Alex Shigo, will encourge soil fungi evelopment and soil fungi tend to make soil pH lower.
This material will get between your clay soil particles, seperate them, and improve the tilth greatly, though.

The pH scale is logrithmic, and each division is 10 times different than the previous on, ie. at pH 7.0 you have no free radical hydrogen ions running around in your soil, but at pH 6.0 there would be 100 times as many free radical hydrogen ions running around.

100 times nothing is nothing. At a pH of 7 the concentration of OH (-) and H (+) is equal. At a pH of 6, there are 10 (not 100) times more H(+) ion than OH(-). At a pH of 5 it's 100 etc. This is from memory but you get the point. In organic lawn care, I don't worry about the pH of my soil, but rather worry more about adding organic material to buffer pH, mitigate compaction, etc.

Just to throw it out there...

On my lot in the middle of the Central Basin region of Middle Tennessee I have approximately 12-16" of very compacted, red clay atop "rock shelves" of various types of bedrock.

Growing into that soil (I use the term loosely, no pun intended) I have several old Pin Oaks and at least two dozen cedar trees. The average pH of my soil was 4.5-5.0 until I started amending it with various types of lime treatments.

So a reading of 4.2 doesn't seem all that outrageous to me at all. <shrug>