White smoke is generally the sign of either a faulty head gasket, cracked cylinder head. White smoke is generally a sign that there is water coming into the combustion chamber.
Oil will produce blue smoke.
You said you added 19qt. of oil in 4,200mi.. This is really bad.
Must be a leak somewhere.
That kind of oil consumption is probaly not a valve problem. Most likely a broken piston ring, or broken piston.
Fist thing I would do is check for leaks, second, take a compression test. With that kind of oil consumption, your spark plugs must be clogged with oil. Check this when taking the compression test. One or a few spark plugs will show you where the problem come from.
Keep us posted on your findings.