99 V70XC P1356 code
 

My 99 V70XC keeps throwing the P1356 code. I swapped out a coil pack from another cylinder, and it still threw it on the same (#4) cylinder. I finally purchased a new coil pack and installed on it, and after about 150 miles, it threw the same code again. Could it be something besides the coil pack on #4 cylinder?

By the way, the car has and does run fine....no obvious "miss".

Any ideas or help is greatly appreciated!!