I just replaced my head, and i seem to be getting the code for a cylinder 4 misfire. Sometimes when im in 3-4 gear going past 35-45k RPM my motor starts to sputter which makes the car not accelerate anymore and the check engine light flashed, until i slow down alot.

I swapped out my injectors a few months back and even changed the coil packs (all around the same time i got a new head)..and the problem seems to still persist. Could it be a spark plug or something else?

Thanks

 

Could be a plug, pull it out and look at it. There are charts on the internet to aid you in identifying issues with plugs.

 

... then look up CEL codes, then compression test. My #1 guess is something was not reinstalled/torqued correctly; possibly [I hope not] the head gasket.

 

You'll likely only ever get 1 code for a misfire, and that will say its misfiring.

 

also, change the spark plugs/coil from cylinder 4 to cylinder 3. if it changes to cylinder 3 you just solved your problem.