Great, glad it's fixed. So that must have happened upon reassembly. From a heater core or by-pass hose before making the final connections. Do you recall spilling some coolant at any time ? It doesn't much lol.

 

I have had a P0304 code multiple times on my '98 4.6. Right above the #4 chamber (passenger side, back plug) is a hose that comes out of the heat exchange in the dash and clamps on to a tube that runs down into the block.

If the hose is lose or getting worn out, it will leak anti-freeze into the #4 plug well. Once enough builds up, it will start misfiring. The plug boot can look clean too. Make sure to pull the plug and dry it off too.

 

Correct, common problem. Ford didn't so much as provide the right clamps at the connection there. Deal is, they used standard worm clamps on the heater hose and heater core hard line tube connection. Hose wears right at the connection. However, you get one cut back. You can cut the damaged end off just once, clean the metal pipe end well. Reconnect the hose but use expansion heat clamps vs standard worm clamps. They are made for this type of connection, retaining a tight and equalized seal throughout the heat cycle.