To start with the 40107 is a MKIV 22/45 target it has a bull barrel, adjustable sights, and a 1911 grip angle (it can use standard 1911 grips. it was designed for those of us who shoot and like 1911s. The 40104 is a standard Ruger MK IV the grip angle is like a German Luger it has a tapered barrel and fixed sights. They are both great guns they will both be super accurate and reliable i have guns in both series and they get shot a lot The MK will be a Aluminum frame the 22/45 will be a poly frame

The only distinguishing difference is the Bull barrel (Target) of the 40107 verses the Tapered barrel of the 40104. The core pistol is the save save a few cosmetic differences.

Barrel lenghts are different too. I put the specs for both below.

The Ruger Mark IV Standard is the iconic .22 LR pistol with modern updates. The new simple one button takedown for quick and easy field stripping and proper chamber to muzzle cleaning is a welcome addition to the platform. Additionally the Mark IV standard features a one piece precision CNC machined aluminum grip frame which is lightweight and durable. The smoothly contoured, comfortable grip features a natural pointing grip angle with replaceable checkered grip panels. Do yourself a favor and pick up one of these iconic modern plinker's today!
Specifications and Features:
Ruger Mark IV Standard Semi Auto Pistol 40104
.22 Long Rifle
4.75" Barrel
Tapered Barrel Style
1:16" Right Hand Twist
10 Rounds
Checkered Synthetic Grip
Fixed Front/Rear Sights
Aluminum Grip Frame
Simple One Button Takedown for Quick and Easy Field Stripping
Recessed Button in Back of the frame allows the upper receiver to tilt
One Piece Precision CNC Machined Grip Frame
Ergonomic Bolt Stop
Internal Cylindrical Bolt
Contoured Ejection port
Magazine Disconnect
Overall Height 5.40"
Overall Length 9"
Overall Width 1.20"
Blued Finish

The Ruger Mark IV 22/45 semi automatic pistol features some new upgrades that people have been waiting for. Starting off, the Mark IV 22/45 features a new simple one button takedown for quick and easy field stripping. A recessed button in the back of the frame allows the upper receiver to tilt up and off of the grip frame without the use of tools. A new contoured ejection port and easy to grasp bolt ears allow for durable and reliable operation. The grip frame is made from high quality impact resistant polymer which keeps the overall weight down. All of this is backed by Ruger who is one of the nations leading manufacturers of rugged, reliable firearms for the commercial sporting market.
Specifications and Features:
Ruger Mark IV 22/45 Semi Automatic Pistol 40107
.22 Long Rifle
5.50" Bull Barrel
1:16" Right Hand Twist Rate
6 Groove Rifling
10 Round Magazine
Checkered Synthetic Grips
Fixed Front Sight
Adjustable Rear Sight
Polymer Grip Frame
Simple One button Takedown
Upper Receiver Tilt's up and off the grip frame without the use of tools
Drilled and Tapped for Weaver/Picatinny Style Rail
Ambidextrous manual safety
Magazine Disconnect Feature
Overall Length 9.75"
Overall Width 1.22"
Overall Height 5.50"
Overall Weight 34.4 oz
Matte Black Finish

The grips are different. 40104 = Checkered Synthetic Grips 40107= Polymer Grip and is also Drilled and Tapped for Weaver/Picatinny Style Rail