I would like to thank all the nice folks for the good advices and
suggestions. Gary was right, I should test the valve adjustment first. This
is exactly what I did. All the valves on cylinders 4 and 3 are fine, the
rocker arm and lifter are tight, and I readjusted them one turn back. On
cylinders 1 and 2 on the passenger side of the engine, I found two very
spongy lifters, I can even use my finger to push the pushrod down but it
bounce back. This probably mean that the lifter is fine, but there are some
air trapped inside (anyone please let me know if this is not what I thought).
One of the lifter was a little spongy. I adjusted all valve one turn back. I
put the valve cover back on, and started the engine, she run really noisy
first for one minute then the loud tapping noise predominate just like what
it sound before the valve adjustment. I let it ran for 10-15 minutes and
occasionally increase the engine speed a few times. The tapping noise did not
go away. I then decided to take a short drive. The noise reduced
significantly just for a short drive out of my driveway. but the noise did
not go away until I drove around a few times. The engine was completely warm
up then, and the tapping noise was completely gone.
Now, what should I do about the spongy lifters? Now that the tapping noise is
gone, does it mean that air in the lifter has been expelled and everything is
fine? Should I readjust the valve one more time after the engine cool down?
I have another question, when I took the spark plugs out, the two on
cylinders 3 and 4 have oil on the tread of the spark plug but electrodes are
clear. Also when I took the valve cover out on that side (driver side) oil
was dripping quite a bit, but not when I removed the valve cover on the
passenger side. Should that be a concern?
Also, the valve train  appears to  very dirty and gummy, should I spray
cleaner like brake parts cleaner to clean the entire area?

Thanks again for all the helps.

Albert