The 4 shot Police magazine spring is even stiffer then the standard Express or Wingmaster mag spring.
The reason for the stronger springs is to insure correct feed under recoil.
Some people practice with the 870 to the point where they can operate it VERY fast.
What some have found is failures to feed a shell into the chamber.
They assume the gun is dirty or defective.
What's actually happening is that when the gun recoils, especially with buckshot or slugs is inertia.
The heavy load causes the gun to recoil harder.
When it recoils the shells in the magazine tube slide forward in the magazine. (Actually the gun slides back and inertia causes the shells to try to remain stationary).
A fast operator can cycle the action BEFORE the magazine spring can push the shells back into the feed position and the action fails to feed.
To prevent this, the 870, especially the Police models, have stronger springs to force the shells back in the tube faster.