Hi!
UE4’s equivalent of a pole vector is called the IK’s ‘Joint Target’. Just like the settings for the End Effector, you can specify a point in the space of a bone in your skeleton. So, in the case of an arm IK setup, you would select Bone Space for the Joint Target, pick the lower arm/elbow joint, and then give it some offset from the elbow (like a pole vector would have). Same thing with a leg, you could specify Bone Space, pick the lower leg/knee, and then give it an offset to move it a bit in front of the knee. Again, that will put it right where a pole vector would be in a Maya rig.
Here’s a screengrab of what a leg IK setup might look like:
Good luck, and if any of that is unclear, just yell.