I'm going to replace the plugs on my 05 4.0L. Right now it has Motorcraft SP440 plugs. I'm looking online for some, but most I see are SP440X plugs.

Does anyone know the difference between SP440 and SP 440X?

 

You want the SP440, looks like SP440x is the same, just updated part number

Ford actually used 2 different spark plugs at the factory in all its Waste Spark systems(coil packs), one bank got single platinum tip(3) and the other bank single platinum centers(3)
Saves a few dollars that way, but you should used double platinum