Rifled slugs are to be used in smooth bore shotguns, you could add a rifled choke tube but that is not necessary. Modified (.20”), is the most constrictive choke you should use with slugs, and never use a FULL choke.
You can go larger diameter ie; Light Modified (.15”), Improved Cylinder (.10”), Skeet (.005”) or Cylinder Bore (.000”).
Sabot rounds are for FULLY rifles barrels, which the Benelli M4 is not.
Also check you choke tube often and make sure it is tight! Remove and clean often and make sure you use a light grease or an anti seize lubricant on the threads so the choke tube won’t seize up. Some shooters have a problem with the choke tube loosening up quite often and the anti seize or a small amount of Teflon tape will help with that.
As far as accuracy goes, each barrel/gun combination likes what it likes. Try different brands of ammo and see what your weapon happy…
http://www.briley.com/boresizesandconstrictions.aspx