Not sure which planet 120 seconds gives you four miles on between heavies. How we acheive four miles is by launching them rwy length apart. That's roughly 4000m but the temperature has an effect on that. You also have to take A/C types and performance into account but that's our job.
Gonzo, I think you'll find that they will have to further refine the 4 mile requirement because it s as unrealistic in your environment as it is in ours.