A four inch skinny would be an interesting bait. The standard four inch is a 0.375 bait. Same diameter as the 3.0 inch regular and 3.5 inch. The 3 inch skinny ( and 2.5 inch ) have a diameter of 0.3125. Unless Jason slid one by me; there is only one diameter of 4" Carrot. Most of the CCM Carrots have a 70% to 75% tapered tail and 25% to 30% body. ( The exception is the "standard" 3" skinny; it has a 1.25" body and 1.75" tapered tail ). The four inch has a 1" body and a 3" tail. I have not "studied" the 5" and up carrots. These proportions create a very "lifelike" bait profile to me. The longer the tail vs body , the more "wiggle" in the bait ( all else being equal ). The smaller the diameter ( given the same body vs tail lengths ) yields the same results.
Jason was good enough to craft me a 3" skinny carrot with a 0.75" body and a 2.25" tapered tail a while ago. This carrot has the same relative body percent set up as the 3 inch regular carrot. Shot from the same plastic, the skinny has a good bit more "action" than the regular. The picture I posted of the box of 3" Skinnys shows this mold set up. The baits are very minnow like to me.
Now, with a 4 inch, I would be very tempted to just go to a 0.3125 diameter and leave it 1.00" body and 3.00" tail. More action would be a 0.75" Body and 3.25" Tail. Any shorter body than 0.75" I would be afraid of where the hook would exit the bait. Now if you were planning on just nose hooking the bait and really getting funky, a 0.3" head and 3.6" tail. Shot in soft this would be a real wiggler. I would shoot the bait in soft, cut it off at 3.65", then re-shoot the head in medium or medium hard of a different color. ( not that I have thought about this at all )