The frame of reference is action sport shooting (e.g. IDPA or IPSC). I will be knocking over steel plate and/or poppers at distances of about 15m. The points raised by CV32 are valid. A modified choke or less will suffice with 7.5. At distances beyond 25m it is my impression that you will have trouble knocking over steel targets.
Not sure you'll ever come across stages with steel at 25+ meters (unless its a slug stage)
Even 20 is pushing it, mostly due to range/bay limitations at most clubs.
In IPSC, all bird shot courses of fire are supposed to be set up so that you can knock everything down using a cylinder bore and ammunition that at least meets the minimum power factor.
So theoretically the answer to your question is, cylinder choke and ammo at 480 power factor or greater.
In reality shooting a stage by getting into perfect perpendicular position to the steel face and getting as close as is safely possible is not the fastest way to shoot a stage so you'll need to experiment with chokes and ammo to learn what patterns best for you and what will or won't knock down steel at the different distances and angles you'll find in a match.
The choke that seems to get the most use in my gun is Light Modified.
If there are tight penalty targets I may choke up a bit
If everything is wide open and fairly close, skeet or cylinder choke.
I'm also using a 26" barrel so my average velocity will be higher than yours with your short barrel.
Pellet size is less relevant than your pattern, but 7.5 or 8 will work fine, 1oz or 1 1/8, 1200 fps (i.e target loads you'll find anywhere)
With your M4 clone, the ghost ring sights will slow you down to the point that you are aiming like a rifle vs a shotgun, and your short barrel muzzle velocity will be slower so you'll need a few more pellets on target for the heavier set poppers or longer distance plates, so you may as well run a modified choke and whatever target ammo you like.
For those "hoser stages" with all targets nice and close or shooting clays vs plates, cylinder or improved cylinder and hose away.
Should do the job for you until you get more experience and figure out what combo you personally like best.