Pressure test and leak down test the radiator. You coukd leakdown test the cylinders.
Factory spec for compression is150 +/- 15. Anything under 130 can be an issue.
The spec for oil pressure for the 4.0 is as follows:
*Not less than 13 psi at HOT idle
*Between 37-75 psi over 1600 rpms
So on#3 do both a dry compression and wet compression test. That will show you if it is the piston rings or a valve.
The things I look at are easy. Air filter. What did your spark plugs look like?? Pics esp #3. Clean TB and IAC. Clean battery connections and the opposite end of the grounds. At the engine and body.
Run some fuel injector cleaner. Run the engine above 3300 rpm for a bit so the valves may move and keep carbon from building up.
Run seafoam or mist water to clean carbon.
What type of gas?? If it has high ethanol you get poorer mpg.
What mpg do you get now?
There is a app pure gas will show stations with no ethanol gas.
How is your transmission fluid?
Any vacuum or exhaust leaks?
Tires all st proper pressure?
Load: what junk is in your trunk?
There are two methods to check CATs.
Before this any Check engine codes?
First mentioned above is a OBDII system that can read O2 sensor info. There you validate the timing of the sensors. Pre CAT should be different from post CAT.
The second is physical. Knock on the cat and listen for anything rolling around. Broken CAT might sound like a loose stone in there. Hollow is just that. The last is temperature. When the vehicle is at temp the front of the CAT should be.200 temp compared to the exit of the CAT app 400 deg
Being this is a 2004 check your OPDA for wear. If a real issue you get screaming monkeys or CEL for CKP snd CPS out of sync
What oil filter brand do you use?
Never look down on anyone unless you are helping them up - Jesse Jackson