I mean all that would really be needed is a mode that highlights all areas that are Radiation Level 4 or higher for the selected unit. Everything else is really of little to no concern.
I beg to differ.
Having played on an extremely radioactive heavily forested world with radioactive megacities and forests, where apart from the cluster of three zones running south-to-north from the southern border, all mostly mildly radioactive (1-3) but having some non-radioactive land, I had to move though huge areas of radiation 200-600 in all directions, with arachnids both to the east and the west and a major to the north, what you are suggesting would be an improvement on the current situation of having no radioactivity mapmode, but at the same time a missed opportunity.
The hits to readiness and morale from lowlevel radiation is not irrelevant when you are having to fight your way through such areas in the early game - at least not on hard difficulty; Later on with better tech and skilled commanders is a different matter.
Likewise, it makes a huge difference when waging war with fronts slowly pushing through the glowing remains of cities whether you are "merely" suffering level 4 radiation, 5, or 6. A huge difference. Being able to see from the map layer at a glance how radioactive areas are through colourcoding will allow the player to make informed strategic decisions about where to fight, where to move quickly, and perhaps, where to give up until they've got better tech.