Hi everyone! I’m new to Unreal Engine 4, I’d really appreciate a hand. The game will feature parkour, grappling, (a lot like swinging like Spider-Man) advanced combat like in Batman: Arkham Knight, and a big open world. (That I can do by myself.) Any help, again, would be really appreciated, and also, how do you change the default mannequin? And if you mind, is there an official download for UDK, or Unreal Engine 2.0? My PC is really bad. Tyvm for your time! 
maybe your being slightly too ambitious since your just starting out ? plus creating a large open world on a "bad " pc is going to cause many headaches
Well, update: I have a beastly rig. Need to add parkour. (First download UE4, I have a bad Wi-Fi.)
Feel free to use my free template to get you started on Parkour (climbing, swinging, sliding, swimming, etc).
Download HERE]([FREE] ThirdPerson Action Platformer - Programming & Scripting - Epic Developer Community Forums)
However, I agree with Starfire that it might be too ambitious to try to get all those things included in your first game.
Good luck!