NGK plugs.

4 is quite a hot plug in the NGK range, I would try them if the price is good. Factory rated they they were a 5 or 6 heat range.
I use 4s in my 455 Pontiac, & have them in a friend's 455 that will get drag raced; sizable cam in that engine & it noticeably improved the idle quality.
In my opinion, many engines have plugs that are too cold..... They they then foul up at idle.