89 L98 TPI Rev counter not reading over 4krpm
 

I recently fitted Accel ceramic plug wires to my 89 with long tube headers and the car is running much better. However, at high RPM the tach flicks around 3600-3900ish even though it's obviously revving higher. Could I have disturbed something by the dizzy when fitting the wires? Or is it time for a new coil? Last time I had a similar problem my coil was on the way out.

If it is the coil, is the DUI uprated coil worth it? I'll be fitting a Miniram soon.

I wouldn't go with more than a stock coil for a stock engine, but depending on the build for the MR, i'd consider upgrading to other HEI-capable coils. Accel, MSD, etc.

Sounds like you may have a tach filter issue, it wouldn't be the plug wires. A scanner would show what the RPM actually is vs what the dash reads to check.