Another point of clarification on my part. First off I was 'victimized' by the infamous 4.0 engine cracked head outbreak of 2000-2001.
Supposedly Chrysler fixed any heads that cracked while under their regular warranty back then, but did not admit to any problem and didn't fix any that were out of warranty as far as I know.
So did they used junk heads again in 2004? I haven't read anything to indicate that they did. BUT you should check your head anyway since it is an extremely long head and is a good candidate to crack if torqued down improperly or had a thin casting between #3 & #4 cylinders.
Or, you blew a head gasket.
The stealership will rape you to replace the head gasket. Find a local Jeep mechanic and make sure you do not re-use the head bolts.
They may have been the problem since they are supposed to hold the head down tight enough to prevent the head gasket from blowing.
Did this engine ever overheat? That could have made this possible.
Once the head is off then the cylinders can be checked also. The other possibility is a cylinder wall cracked.
Do you know what was good about the good old days? I wasn't good, And I wasn't old!
Senators and Congressman should wear uniforms like NASCAR drivers so we could identify their corporate sponsors....