I keep getting a # 4 cylinder misfire, I have changed the plugs, wires, coil and even the fuel injector. Any ideas?

Low cylinder pressure, defective fuel injector or fuel injector wiring harness, defective Powertrain control computer, intake manifold leaks vacuum.

If it has a coil for each cylinder then it could be the coil on that cylinder but if it doesn't then it could be plug wire,sparkplug or fuel injector related ( injector wire harness or pigtail ).

the bit you missed is the injector
sea foam through anything but a fuel tank is a waste of money
get that injector fixed

so missfire detected and cylinder 3 missfire detected.
check to see injector pulse with "noide light" if ok injector is problem
remove fuel rail and injectors look into top of injector for blockage in screen filter in injector if blocked clean out or injector carby places can remove this screen replace it and test injectors and clean or just buy new injectors. make sure to clean out fuel rail and change fuel filter also.
if noide light flashes irratic or not at all wiring issue, check for power on one wire with ignition on 12v other is earthed by ecu when opened to inject fuel. other thing that can cause problem on cylinder is tappet clearance "shims" too tight, could do compression test to see if compression is low on cylinder or a cylinder leakage test to see if valves are leaking. broken valve spring. valves do not hit piston on these engines even if cam belt breaks, they are a "free wheeling" engine

Have you compression tested the cylinder to make sure it is in mechanical working order? :)

Fire it up at night and look for arcing in the dark. It may also be nothing more than number 1 plug wire misrouted in a way that inductively picks up energy from one of the other coil packs or cylinders. It should cross other plug wires only once.

Plugs get changed at 70,000 miles
I would suggest that you use the OEM
Plugs,they are made by NGK and are special
on my 2004 LS,I don't know about yours.

Clean the MAF Sensor with the CRC Product
for that

Change the 2 front O2 Sensors at 100,000 miles

Check fuel pressure at the fuel rail on engine

The coil you replaced on #6 ,even though you
have a PO306 Code,doesn't mean that was the
bad one. It is a COP Coil or Ford thing, you will
learn as you get involved with them

In some cases, you buy 1 new coil and walk it
from one cylinder to the next.

On single coil codes, you may get lucky,on multiple
codes or a really bad coil you won't

Your not using any fancy test equipment and
unfortunately a lot of repair shops have it and
don't know how to use it,including dealers

V6 or V8 ?

P0301 - Cylinder 1 Misfire Detected.

Causes:
- Improper spark plug at cylinder #1
- Insufficient compression at cylinder #1
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #1
- Fuel injector #1 may be faulty
- Intake air leak

Solutions:
- Perform Injector Service
- Replaced Spark Plug #1
- Replaced Injector #1
- Replaced Ignition Coil #1

P0303 - Cylinder 3 Misfire Detected.
Follow info from P0301.

P0316 - Misfire Occurred In The First 1,000 Engine Revolutions.

Causes:
- Damaged CKP sensor.
- Ignition System.
- Fuel Injectors
- Running out of fuel
- Fuel Quality
- Base engine
- Damaged Powertrain Control Module

Solutions:
Usually the P0316 is combine with a P03XX codes. The P03XX code indicates in which cylinder the misfire occurred.

i have a 2001 chevy z-71 5.3L. I keep getting a random/multiple misfire code for cylinder 1 . I have replaced plugs, wires, coil pack(only on #1 cylinder)., and fuel pump. I still keep getting the cylinder 1 misfire code. I put truck on a scanner and watched the cylinders. # 1 misfires at idle but quickly stops misfiring at around 1000 rpms. you can here the engine miss at idle but truck runs great on road . anyone else ever had this problem?