Yup, the walking through (if without sudden teleport) is quite complex. In 3DGamestudio I implemented a custom Trace-Method that continued to trace when it hit a PortalSurface(it was translated through the portal and continued from the other one). That was quite simple as the engine supports PhysX, but it’s mostly not used. However, in UE4 ALL Collision-Queries are by PhysX, so i need to find a way to implement/override functionality. And that’s something i don’t know how to do atm.
Oh and for the Seamless portalmovement:
You need a volume/Box behind a portal-surface that continues to render portals color on to the screen, where the camera clips through the surface. Otherwhise you can look behind the portal while walking through(and see a wall or whatever)
Clones: All objects (atleast in my design) get a clone of themself. clone is translated according to the Portals as you need two objects to have something stuck in mid portal(One half is seen at PortalA, the other half sticks out at PortalB).
And clipplanes: Clipplanes Clipplanes Clipplanes!
That’s where all the anoying stuff starts 
EDIT:
Oh and i hope you created two (BluePrint)Functions to do Location or Rotation transformation between 2 portals. (I translated mine from C (Old Engine) to BluePrint). You’ll want to transform a lot of different things between portals!
Greetings