Truck started running rough all the sudden. Clearly misfiring and rotten egg smell from exhaust. Pulled codes P0301 and P0304. Replaced corresponding plugs. Cyl 4 still missing. Replaced the wire on cyl 4. Cyl 4 still missing. Did an "old school" spark test on cyl 4, grounded the plug to intake, started the truck and saw spark. At a bit of a loss here. Anyone have this issues or one similar?

 

Put spark plugs back in and disconnect coil wire harness, so you will have a no start.
Crank engine a few times.
Pull out #4 and #5 spark plugs both should be wet with fuel, if #5 is but not #4 then fuel injector issue.
You have tested both spark and fuel now so only think left is compression

So next would be compression test
All spark plugs removed
Install screw in compression gauge on #1
Crank engine so you hear compression stroke hit 4 or 5 times on #1
WRITE IT DOWN

Move to #2
repeat for all cylinders

1999 3.0l should show about 165psi or higher with good battery(crank speed) and accurate gauge.

But compression test is really about the comparison of psi from each cylinder.
If 4 cylinders are 160-170psi and #4 is 120psi then you have a compression problem.


And just because, recheck firing order on Coil Pack
the 5 6 4 on the one side can get mixed up
You can also swap #3 and #4 wires on the coil pack, if problem moves to #3 then replace coil pack

Coil pack
3 4
2 6
1 5
front
There are only 3 coils in this coil pack
3 and 4 are on the same coil
as are 2 and 6, and 1 and 5

It is called a Waste spark system, so 2 spark plugs fire at the same time, one spark is wasted on the exhaust stroke on one cylinder.
Spark plugs are wire in series.