Thought I would post an update for posterity. The 10K ohm resistor mod illustrated here works just dandy on the I4 tach as well - but the tach reads about 33% high. No worries, this places the reading within the adjustment range of the tach. My tach now reads, and does so accurately.

FWIW, this would also imply that there is no reason for those who want an SR5 cluster to limit themselves to the same cylinder count. Got an I4 but found a V6 cluster, or vice-versa? I'd bet you can adjust it. The easiest way is to temporarily remove the speedo, attach the harness to the cluster, and adjust the small white potentiometer on the circuit board of the tach. Oh, best to use a plastic TV tuner adjuster, you don't want to touch any wires/traces with the metal tip of a screwdriver. Second option, which is what I did, is to put shrink tube on your long small screwdriver and aim carefully.