I would like to make a game that has both an online team death match and a open world story (a bit like assaying creed origins) is that possible to make on unreal engine 4
Short answer yes.
Long answer:
Unreal Engine is mostly specialized in making a 3D shooters. Everything else is possible(with enough c++ knowledge certainly) but the more you move away from the traditional unreal tournament gameplay the more difficulties might you encounter.
Both an open world story mode and a team death are close enough to unreals specialization to make the engine suitable, however “open world stroy mode” requires alot of content creation, which means many artists while “team death” requires pvp-balancing, multiplayer and network coding, which is a science on its own.
My point being: the proposed undertaking sounds like a massive project. The Unreal engine is truly capable of such a project, however since you need to ask this question I suspect you should first try some smaller projects to learn about the engine before you approach a triple-A sized game(which is usually done by a double to triple digit crew).
If you still want to be working towards your goal then I suggest you create feature-prototypes. Pick a single part of the game, for example walking around together as 4 people in the same world and implement that in a project.
Then create a new project and implement something else there, maybe an NPC that you can talk to… etc.
With ue4 all you need is enough blueprint knowledge, c++ to a new developer is too advanced, they wont have anything close to a prototype starting off with c++. Blueprint is perfect for starters, then if they don’t feel they are have full control of what they want then they get into c++.