Reduce the powder charge in 5 grain increments, until the pattern tightens. Use FFg and not FFFg powder. At 90 grains, you are blowing your patterns, by trying to duplicate Modern shotgun shells.
All that extra velocity is used up in the first 20 yards, and you don't need more velocity shooting #4 shot inside 20 yards.
An Improved Cylinder gun is basically a 25 yard gun, and you can use #6 shot to put more pellets in the pattern. At 25-30 yards, #6 shot pellets will carry enough energy, even at lower MVs to kill a turkey. If you want to stretch the range to 35 yards, then use #5 shot instead.
You might try, in addition to reducing the powder charge, greasing the bore of the gun after the shot and OS Cards are loaded by using a well greased cleaning patch to run down the bore. This will help the shot pellets SLIDE over the barrel, rather than rub flats on the outside pellets, and leave lead streaks in your barrel.
Those lead streaks don't matter much to the FIRST pattern you shoot from a clean barrel, but they help ruin subsequent patterns shot from the barrel if you don't first take the time to use lead solvent, and a bore brush, to scrub the lead off the bore.
If you don't own it, By the most current version of the Lyman Shotshell Reloading Manual, and check the tables on MV, down range velocity, pellet energy, time of flight, and drop in flight. The tables give data for loads from the muzzle out to 60 yards, for modern guns. But you will learn a lot about shotgun ballistics, by comparing the fastest loads, to the slowest loads at the various distances. Since all lead shot is round, and has the same drag factor( ballistic's Coefficient), it doesn't matter if you shoot pellets out of a modern shotgun shell, or a BP ML shotgun. Velocity will be lower, because BP is not going to generate the kind of velocities you can get from Smokeless powders. But the affect that air has on the pellets is the same.
Keep the velocity of your load below the speed of sound to improve patterns, too. SOS is 1135 fps, but the transonic zone ranges from about 1080 fps, to 1200 fps. In that zone, air is buffetting the round pellets from all sides, and any imperfection in the surface of the pellet will cause it to veer away from the line of fire.( ie. Blowing the pattern!)